It's a very rare "Curse" with unknown origins that happens without a source casting it. Getting bloodmarked happens when a person is full of resolve and fights untill he/she is simply 'beyond' death, but keeps living. Bloodmarked get a weird change on their body, For example, A weird mark somewhere on the body. And something less noticable like slight change in natural hair, skin or eye color.
There are Seven "known" Bloodmarked in legends. But characterics were never given. Bloodmarked do not seem to be able to get properly reanimated.