Well I guess that you should get something that is like black copper but with lot more buff/brown with sweetie in F1.
That is always a hard one to predict. It;s because there are two main genetic blacks- one is Extended black, it makes the bird pretty black all by itself.. many hens even are solid colored. Here, a good example are BC Marans, many roosters with color limited to hackles, wing patch or a minimal amount on saddles. Crosses with Extended black tend to be pretty black, even with a black tail buff.
The other common kind of black is Birchen. They have a lot more color.. crosses with a buff potentially could give birds with a lot more "color" especially if they have other certain genes like Pg and Db...
a great example of black crossed with colored chickens are black sex links- which are most often a cross of RIR with barred rocks.