If the banana skins are completely black they may well be off. In which case its probably not worth using them. I have used bananas that have skins ranging from pale yellow to brown spotty and find that these always work well. Bananas with brown spotty skins will be softer and so will mash up easier, but the taste isn't really affected.  Bananas with green skins are too underripe and will be very hard and less sweet. To speed up the ripening process, loosely wrap the bananas in a brown paper bag.