This project is a great example.
I can't really speak for others but seemed like the goal was to take the legbar which has some great qualities like the blue eggs and autosexing and add a new color/pattern.
It started with a legbar crossed with a leghorn.
Just keep the legbar pretty much intact and bring in the lavender from the leghorn.
With projects you figure out what you want, what you need to use to get what you want , what it will take to get there and what it will take to breed true.
There can be more then one way to get there which makes it fun.
I know for some it was to get that lavender in the wild type pattern and keep the autosexing. So it was a matter of bringing in the lavender and also get the barring on the same bird. Then to the next step of getting males double barred so they would breed true and be autosexing.
Color/pattern was the focus and bring the blue egg genes along for the ride.
I saw it different. My thought was the color was easy or for me color is easy. From the start I saw color secondary with priority on egg color as I saw that as the biggest challenge. I started the same project but just because of the challenge of egg genes I scrapped my project to focus on others.
I know one breeder crossed the two breeds so they had offspring that had the duckwing pattern, barring and were split for lavender as well as split for blue eggs.
They bred for color so crossed offspring back to the leghorn so the offspring would be 50/50 lavender/split for lavender. That got the highest percentage of lavender and then all that was needed was to bring the second barring gene to the males to complete the project.
I would of "bred for the blue egg genes". I would of crossed the offspring chicks together. That only gives 25% with lavender but it would of give 25% offspring with two genes for blue eggs, 50% with one blue gene and 25% with only white egg genes.
I would hatch as many as I could and kept what I could use color/pattern wise but focused on egg color.
Same as doing the other way is wait till they sat after laying. Some would lay white eggs so those would go. Others would of been a mix of blue egg layers. 1/3 of those would have two genes for blue and 2/3 would have one gene but lay blue eggs.
In theory the ones would only one gene should lay a lighter blue egg so if check for deepness of blue and move forward with just the darkest blue egg layers.
The other way of crossing back to the leghorn would give more lavender but with the offspring split for blue/white eggs crossed back to the leghorn would produce 50% with white only genes and 50% with one of each gene.
You would not get the mix of both kinds of blue to maybe be able to compare them and you would be moving forward with just split egg gene birds so still not getting anything that could breed true for all blue eggs.
My next step would be to continue on with test breeding just to be sure and since roosters don't lay its all you can do.
Is separate everyone so I could keep track and breed to leghorns or any pure for white eggs.
If you hatch enough and get any offspring that lay white then if know that parent carried a white egg gene. Since blue is dominate a parent with two blue genes would produce all chicks laying blue eggs when crossed with white egg breed.
I could always pick up the lavender color along the way. It would probably take longer and id have to be more careful because is end up with split for lavender and some that didn't carry lavender at all that I couldn't tell about but I should get enough lavender with enough chicks hatched to make progress.
My way would be breeding for blue egg genes.
As you see it would be a bit of a pain with hatching a lot of chicks and waiting for a couple generations to get to laying age and then test breeding and keeping those to laying age.
That might make sense of why I decided it was more then I wanted to continue with at that time.
Also you can see how the same project can go different ways and end up at the same place.
For me worrying with color more then egg genes would stretch it out a lot longer by going back to leghorns and back to white egg genes.
You have to get pure for blue eggs to breed true for them so that has to be an end goal however you go about it to have this project be successful.
I can't really speak for others but seemed like the goal was to take the legbar which has some great qualities like the blue eggs and autosexing and add a new color/pattern.
It started with a legbar crossed with a leghorn.
Just keep the legbar pretty much intact and bring in the lavender from the leghorn.
With projects you figure out what you want, what you need to use to get what you want , what it will take to get there and what it will take to breed true.
There can be more then one way to get there which makes it fun.
I know for some it was to get that lavender in the wild type pattern and keep the autosexing. So it was a matter of bringing in the lavender and also get the barring on the same bird. Then to the next step of getting males double barred so they would breed true and be autosexing.
Color/pattern was the focus and bring the blue egg genes along for the ride.
I saw it different. My thought was the color was easy or for me color is easy. From the start I saw color secondary with priority on egg color as I saw that as the biggest challenge. I started the same project but just because of the challenge of egg genes I scrapped my project to focus on others.
I know one breeder crossed the two breeds so they had offspring that had the duckwing pattern, barring and were split for lavender as well as split for blue eggs.
They bred for color so crossed offspring back to the leghorn so the offspring would be 50/50 lavender/split for lavender. That got the highest percentage of lavender and then all that was needed was to bring the second barring gene to the males to complete the project.
I would of "bred for the blue egg genes". I would of crossed the offspring chicks together. That only gives 25% with lavender but it would of give 25% offspring with two genes for blue eggs, 50% with one blue gene and 25% with only white egg genes.
I would hatch as many as I could and kept what I could use color/pattern wise but focused on egg color.
Same as doing the other way is wait till they sat after laying. Some would lay white eggs so those would go. Others would of been a mix of blue egg layers. 1/3 of those would have two genes for blue and 2/3 would have one gene but lay blue eggs.
In theory the ones would only one gene should lay a lighter blue egg so if check for deepness of blue and move forward with just the darkest blue egg layers.
The other way of crossing back to the leghorn would give more lavender but with the offspring split for blue/white eggs crossed back to the leghorn would produce 50% with white only genes and 50% with one of each gene.
You would not get the mix of both kinds of blue to maybe be able to compare them and you would be moving forward with just split egg gene birds so still not getting anything that could breed true for all blue eggs.
My next step would be to continue on with test breeding just to be sure and since roosters don't lay its all you can do.
Is separate everyone so I could keep track and breed to leghorns or any pure for white eggs.
If you hatch enough and get any offspring that lay white then if know that parent carried a white egg gene. Since blue is dominate a parent with two blue genes would produce all chicks laying blue eggs when crossed with white egg breed.
I could always pick up the lavender color along the way. It would probably take longer and id have to be more careful because is end up with split for lavender and some that didn't carry lavender at all that I couldn't tell about but I should get enough lavender with enough chicks hatched to make progress.
My way would be breeding for blue egg genes.
As you see it would be a bit of a pain with hatching a lot of chicks and waiting for a couple generations to get to laying age and then test breeding and keeping those to laying age.
That might make sense of why I decided it was more then I wanted to continue with at that time.
Also you can see how the same project can go different ways and end up at the same place.
For me worrying with color more then egg genes would stretch it out a lot longer by going back to leghorns and back to white egg genes.
You have to get pure for blue eggs to breed true for them so that has to be an end goal however you go about it to have this project be successful.
That is a big egg! 
