This is copy/pasted from a forum I used to go to (it's a sticky there).
"this is what I know about this situation, don't know if it's good enough--but it's what i've been told by my doc a few times when i've been puking the stuff up.
there are two types of blood that a bulimic normally types up.
the first is light colored, thin and pretty bright red--this is not very dangerous. usually apears as red streaks in the vomit. it usually means just a surface scratch in the the throat or mouth and that something of a petty scratch. it may hurt and even cause your throat to sting, but still is just a minor problem. however--if the bleeding persists, or there is a lot of blood, it's wise to call your doctor.
the second is thick, dark blood nearly brown or black that appears to be coffee grounds--thick and chunky. it will be metallic tasting so you know it is blood, and may be very dark red if examined closley. this is DANGEROUS. get to an EMERGANCY ROOM if anything resemblaing this is in your vomit. this is internal bleeding from deep in your body, from an internal organ or your ruptured esophagus. this can kill you or deeply hurt you, so act quickly and TELL SOMEONE fast to help you help yourself. you must get medical attention NOW.
so rule of thumb--light is alright, dark and thick means you're really sick.
that's the best i can do."