You can use a wash out color that can be found in drugstores, grocery stores and online. They come in spray-in (usually around Halloween), brush in, and there are also lots of tinted hair gels. Some might not be suitable for dark hair.

The product I really like is a color wand type thing. You mix it together and brush on. It works really well on really dark hair. It comes in a box, and I think i found it at k-mart but I can’t remember who makes it. The other problem is that they don’t make it in blue. (or purple or green.) Something about the color molecules at that end of the spectrum.