Sounds like mites. As far as I know, there are no fleas that infest snakes/geckos. You should probably look very closely around the eyes to see if there are any on the lizard. I don't have and have never had geckos, but I do know that with snakes, you can soak the animal in warm water, just enough to cover half way up the body. If it has mites, they will drop off into the water.

If you decide that they are mites, invest in a can of Provent-a-Mite. Follow the directions to the letter! You should probably treat the snakes while you're at it, because mites do travel. You'll need to treat twice as per the directions. While your letting the new substrate dry, clean the cage THOROUGHLY! Use bleach if the cage is glass or plastic (you don't want the bleach soaking into wood). Throw any wooden cage decorations away and replace them after you're mite free (you might be able to wash them and bake them in the oven, but the best bet is to just chunk 'em).

Good luck!
Steve

edit- BLACK mites are mites that have been feeding and are engorged with blood.