I noticed when I worked with the black fondant that there was a little bit of black on my hands. Once I apply it to the BC frosted cake, will the black bleed on the frosting?
It shouldn't as long as you keep what you are attaching it with light. If you use vanilla or edible glue and get carried away it may bleed then from the moisture under it. But if you use it sparingly you should be fine.
before when I applied fondant on bc I just stuck it on....should I apply something to it first?
No, if it sticks and stays then you don't have to add anything else.