I think understand how the BBS gene works when bred to another BBS chicken but how did the blue from the dad become black when mixed with buff? If it’s too complicated don’t worry about explaining. It’s not a huge deal. Just seems to be an interesting pattern and color that I wasn’t expecting
There are a number of genes that affect how much black and how much gold are on a chicken, and how those colors are arranged. They make the difference between a solid black chicken, or a black laced gold chicken, or a buff chicken, or various other options.
Then there are genes that can change the gold to red, buff, cream, or silver. And there are genes that can change black to blue, splash, chocolate, or white.
Regarding the blue gene specifically, it affects the black parts of the chicken, but does not change where those black parts are. So an all-black chicken would become all blue, but a black laced red chicken would become a blue laced red (the black lacing turns blue, but the red stays red.)
The Buff Orpington probably does not have the blue gene, so it is genetically "black" rather than blue or splash, even though there isn't any visible black to see. Breeding a blue rooster to the buff orpington can give chicks that show black or chicks that show blue, and some of the other genes make it so the black or blue is visible in the feathers (so the chick is not solid buff.)