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.
He must also have a certain Epoch or Radix from whence to derive, first his greater judgment, secondly his lesser judgment, when he intends a serious discourse. - William Lilly (1602.-1681.)