I was wondering the same thing. Several people told me not to use a black from a BBS breeding.
I need to find some blacks or splits for a small project I want to do. Is is because a blue produced could be hard to tell from lavender if it is a lighter color?
a black from a BBS flock is just as pure black as a black from two parents. Black is black. You can get double expression of different dilute genes, blue, or splash over lavender, (or lavender over blue or splash, same difference) and the bird will be double diluted. If you had a lavender splash bird, and crossed it on blacks, ALL the babies would be blues, and ALL the babies would be split to lavender. A blue, genetically blue bird that was a visual lavender bred to a black, half the babies would be blue, half woud be black, and all would be split to lavender. If you never bred for unknown splits, i.e. always working with a viusal lavender over either splits or non-lavender carrying birds, you would know without a doubt that all of your birds either are or carry lavender. Sometimes starting out a person has to breed for unknown splits because they have few options. As soon as they have visual lavenders, it would be best to remove unknown splits from the breeding pens, just so you could always have known results from there forward. At least that is what I would do.
A blue lavender or a splash lavender looks more like blue or lavender than like lavender. On blues you will get darker head & hackles; on splash you will get splashes. There is a way to tell from the tail feathers whether lavender is present or not, but I don't recall the specifics.
While blue always makes its presence known, very dark blues that look almost black exist.
You are completely right about the "almost black" blues. There is just a FAINT wash out of the feathers along the shaft on some birds that otherwise just look black. I have a marans that hatched out like that. i KNOW it is blue, because it came from a pen of blue/splash hens with only a splash roo, so there is only one suspect for Who's the Baby-Daddy game... It is really in the ugly stage right now with feathers sticking out every which way, but later one when it is prettier I will try to remember to post pics.