Blood in a dream commonly represents life force and vitality, so its appearance often signals that something important is at stake or feels like it's being drained from you. Seeing blood without injury can point to unresolved pain finally surfacing, something you've carried quietly that now needs acknowledgment.
Three manner of wayes the Planets are varied or changed, by reason of their first qualities, of their Houses, and significations. - William Lilly (1602.-1681.)