I haven’t done that cross but you’ve picked some interesting colors to work with. My head is spinning just thinking about it.
The simple genetics is the genes that make blue. It’s the Blue, Black, Splash gene. It is not a sex linked gene so both parents have two genes at that location and may give a copy of either of the genes they have at that location. If there are two copies of that gene present you get Splash. If there is one copy of blue and one copy of not-blue, you get a blue chicken. If there are two not-blue copies, you get a black chicken. This gene only affects black feathers so if there are no black feathers present you won’t be able to see it.
To make a solid blue bird you’d think the underlying base color is black using the Extended Black gene. That’s normally the case but it is also possible to make a solid black or blue bird with other genetics, say Birchen with modifiers called melanizers. Your Blue birds are probably based on Extended Black genetics but there is no guarantee of this. I told you my head is spinning and this is the simplest side.
Buff is a lot more complicated. There are different ways to make buff and whichever way yours have takes a combination of different genes. Some of those genes are pretty powerful and can have a strong effect on black feathers. Some of the effect can depend on what the black/blue bird contributes to the genetic cocktail. A lot of things can hide under black/blue. A buff bird could be based on different genetics but I’ll assume Wheaten at the same location that I’ll assume your Black birds have Extended Black. It’s a guess but a reasonable guess.
When you breed crosses you don’t know which genes are going to actually be passed down. The chicks you get are not likely to all look alike. You can get some really different-looking chicks from the same parents with crosses involved.
You Buff/Blue cross chick should have one Wheaten gene and one Extended Black gene at that location. I think that will be the rooster though that should not matter that much. He will give either one randomly to his offspring. The buff hen will have two Wheaten genes so her offspring will get a Wheaten. In theory half the offspring will have two Wheaten and not be black. This does not mean a solid buff/red chick though. It’s quite possible that there are other genetics that will give black feathers in the tail or other locations. If your original Blue chicken has Silver hiding under all that instead of gold you might even wind up with some mostly white chickens.
The ones that are split, have one Wheaten and one Extended Black, should be basically black. But they are going to get a real mix of modifiers. They could wind up looking a lot like your blue/buff mixes or they may look a lot different. It’s unlikely they will be solid black though.
All this assumes that you pick a cross that has black feathers. If you pick a cross that has blue feathers then some of his offspring will have one copy of the blue gene and some will only have the not-blue gene. Where I said black above change that to some will have black and some will have blue.
I told you my head is spinning. To try to sum up, if you cross a black/buff or blue/buff cross rooster with your buff hens I think you will get a pretty wide range of looks in the offspring. Don’t expect a lot of consistency. I also think some of those will have very pretty feathering.
Post 1710 in this thread shows a photo of a pullet from a buff/black cross and there is some discussion on buff and black crosses in the posts after this photo. Your pullets may or may not look like this in bright sunlight but this shows what can happen when you cross black and buff. It can be striking.
https://www.backyardchickens.com/t/261208/sex-linked-information/1700
