The black is usually castor oil + accumulated dirt. (And as a standard dancer... please don't use castor oil, or at least apply sparingly the night before, then blot with a paper towel, then brush. When applied right before dancing, it leaves random sticky spots behind... and when you hit one on a generally slick floor when you're going into, say, a slide, it HURTS. Like, sprained knee hurts.)
A remedy that won't make me want to murder you: brush your shoes well, then step on a damp paper towel. Repeat paper towel stepping as needed. (Bonus points if you place the towel in a container so it doesn't leave a wet spot on the floor. Then, I might actually like you.)
Even better: refine your weight transfer so you are better able to cope with slick floors, because they might happen at comp too.