I believe the black bird does not have the blue gene, period.
But you do not want to mix BBS birds with Black birds. I think it is much preferred to stick with the BBS birds separate.
Going in it wont be so bad, but the results a few generations off may not be so kind to the color.
Like, for instance, a blue bird that looks black, looks like a black bird but doesnt have the beautiful green sheen that true blacks have.
Then you would never be able to take a 'black' bird out of that line and breed it as a black.