With black mollies, you just have to wait and see. One of my black mollies has been pregnant for a few weeks. Since I can't see a gravid spot on her, I have to check the tank a few times a day, looking to see if there are any fry. The only problem I've had with that in the past is that I don't know that she's given birth until she's slimmed down and most of the fry have been eaten. Perhaps there's a better way to tell when they're close to dropping the fry (other than when she gets so huge that her abdomen is square), but I'm not aware of it. Good luck!
