I think you will have to do it frame by frame. As I started reading your post I started thinking, "Oh no, if he had filmed in colour he could have simply applied a colour pass filter and kept the red" But then later you said that the blood wasn't red but black so I think you are down to dopign it frame by frame (rotoscoping it). But you might also be able to get away with keyframing a mask in something like After Effects and then adding the colour to that.

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