If i breed my chestnut mare with a black stallion, what color might the baby be?

You really can never be sure. You could probably rule out a buckskin or palomino, but you could get anywhere from a chestnut to a black or bay. Its all about genetics if you want a buckskin you could breed like a palomino and a black but it doesn't mean you will get it. The dominant color in your instance is the black so the chances of you getting a black or dark bay are stronger than you getting a sorrel or true chestnut. Hope that makes sense!!!