If you mean in their bedding, where they lie down, etc., then you're probably seeing flea larvae (especially as you've mentioned dried blood). You may not see any adult fleas on your cats, as the main bulk of the flea population is in the environment rather than on the animal. Make sure you vacuum their beds, carpet. etc. on a regular basis and use one of the cat spot-on flea treatments from your vet (Frontline, or similar). They should also be on a regular intestinal wormer.
If you're really worried, you can scoop some of these things up in a container and take them to your vet. He/she should be able to identify them for you fairly easily.
Actually, flea larvae can appear black if they've been feeding.