A bird with two blue genes would be a splash and lavender double genes will be lavender so the color comes out as a lavender splash.
The BBS myth was created to sell eggs and chicks. There is no way to visually to tell if the blacks and splashes that are used are homozygous for the pattern genes necessary to produce a good blue. The same with the offsprings that are black and splash from blue to blue breeding. Only from a flock that has been closed bred for several generations that produced top quality blues consistantly would the blacks and splashes be of any use in a breeding program. Otherwise they should be sold as pet quality. Lot of the blacks from these would possibly show a purple sheen instead of the green sheen. Good blues should be silver base and good blacks should be gold/red base. So if you want good blues that are consistant you need to search for a breeder that is successful with this color consistently. IMO
Oh, I do not breed blues.