I have heard of black color variants of Southern toads, so a black American toad is not surprising. I don't know if this is a true melanistic individual or not, but it's a neat-looking toad. It sounds like a happy, healthy toad - I would certainly keep it if I was you!
I think a 10-gallon tank is just fine for a pair of 1.5 inch toads - I have a pair of 4+ inch Woodhouse's toads in a 10-gallon tank and they seem happy to me!