Simplest explanation: black is incompletely dominant over white, making heterozygotes grey. Then, a gray rooster x a black hen would give about 50% black and 50% grey offspring.
Work Step by Step
The numbers of offspring listed in the question are about 1:2:1 black:gray:white, which would make sense if gray is heterozygous, making the first cross in the question one between two heterozygotes. The second cross then is BbxBB, leading to the results listed (try the Punnett square).