Yep, you can really add in any color or strain to help them.
Black is actually a blue bird. What makes it appear black is a gene called 'spread'. Spread is dominant, so it only takes one copy of the gene for it to show. Spread also turns ash-red birds lavender, and brown birds solid brown. If they do not show spread, then they aren't carrying it.
If your black birds that you have now are pure for spread (they have two spread genes), then all their babies will be spread. Doesn't matter if this black bird is a hen or a cock, since either sex can have up to two genes for spread

So that means if you mate one of these pure blacks with a blue bird, then you'll get all black babies. If you mate it with a red bird, you could get blacks and lavenders. Etc.
If the black bird you have is only carrying one gene for spread, then you'll get some spreads and some regular colors.