I'm dyeing my hair black today, with bleach blonde underneath...?

I've done that before, and you could try foiling (wrapping tin foil) around the parts you're bleaching to keep it away from the rest of your hair. When I did it, I first bleached/foiled the underside then dyed the top, and clipped it up to keep it away from the blonde. Make sure not to leave in the bleach too long, or it will fry your hair which sucks.