As far as most things go you can use the Punnent Square.
Your balloon molly is Br (B= balloon r=regular) Capital letters = dominate lower case = recessive
Your black molly is probably Rr (unless he is the result of a balloon molly and regular breeding, but assume he is an Rr)
So 50% should be balloon and 50% should be regular, or close to 50% on each. (Dino could correct me if I did that wrong, but I think it is correct.)
Just so you know, balloon mollies have a stigma attached, and I agree with it. They are bred to be hunch-backed and squished down in length, that compresses their organs and has to cause constant back pain, but I have no proof of that, just so you know in the future. (Whether you agree with that or not is up to you.)
Edit: Sorry, tried to enter the punnent square and it wouldn't work out properly, will try to get a pic of one up this evening.
