Blue Eggs

[Gobbles3]

Has anyone else had their blue chicken eggs become impossible to candle after being in the incubator a week?
I candled the eggs before I put them in and I could see inside fine.
But after 7 days the eggs are impossible to see through and the regular eggs are not like this.
They cannot be rotten because they were fine when I put them in the incubator and were super fresh like picked up the week I put them in.
Just wondering if it's normal

 

[sourland]

They can be difficult, but generally air cell size can be evaluated.