Let's not kid ourselves. It's blood.
Or paint. Or mochi. Very colorful mochi. Besides, according to Reimu in WaHH, youkai don't leave body parts when they die but they simply vanish, so it'd be weird to have blood from some otherworldly being (assuming no humans would have been conveniently pounded in the way meeting the heroines)
At any rate, besides Marisa's injury in WaHH and Kokoro's splatter-ish LW, or some scratches in EoSD, there's hardly any VERY graphic depiction of blood, and the other darker parts of the setting are merely allusions. Maybe the blood-like pattern has been put there by the rabbit for fun, which follow more the prank-ish behavior of the rabbits and the games's light hearted tone.
Canon aside, if this character manages to kick open the dams that'll let loose torrents of rabbits pairings, I'll be very satisfied :V