What will I get?

From playing around with the chicken genetics calculator it seems that black dominates most of the time, so I speculate that you will get black chickens (though if you were to cross some of those back to the buff, or breed them to each other, you would get some buff) but someone may have a better answer.

It will also depend on if either parent is carrying some "hidden" genes. Such as, I have some blue Marans that are from a Wheaten hen, so they carry the wheaten genes.

 

From crosses of buff and black that I have seen, the offspring tend to be a mixture of buff and black, with no particular pattern. Is there a particular reason you want to do this? I am not sure there is anything to be gained from doing this.

 

The result will be on all black chicks that will grow to have a lot of gold leakage on the breasts(look at black sexlinks for a similar color pattern).

 

I did this and got basically black chicks with gold leakage. Not as much buff came through as I'd hoped for.