Black and blues would be my understanding.... MAYBE split to LAV?
Lav to Lav would be Lav.
Lav to anything else should be thought of as black.
Black (Lav) cross blue would equal 50/50 black and blue... in theory.. But I find that to be a very skewed # when taken into consideration with LOW #'s. Meaning hatching 10, maybe you only end up with 1 blue that batch. But over the hundreds or thousands is where those averages become more accurate.
If I had room, I would make 2 pens an keep them pure for breeding... and note that breeding black to blue will eventually end with with really dark blues... But I'm still here to learn!
First generation lavender x blue will result in black and blue, split for lavender. If you breed those back to lavender you will get blue, black, lavender and blue lavender. A bird can be both blue or splash AND lavender. The problem is telling those from the ones that are just blue or splash. Most breeders keep BBS and lavender separate.
A big huge mess is what you get. They are unrelated genes. After a generation or two, you end up with birds that could be Blue/Splash AND Lavender. Makes it impossible to tell which birds are carrying/expressing which genes. Good breeders keep the varieties separate. If you cross them, the offspring becomes worthless to anyone wanting to breed/exhibit.