User blog comment:Kogath/Monster Hunter XX Discussion/@comment-26532315-20161027164501/@comment-27465049-20161028070810

I read that de-oxygenated blood has a bluish colour. Probably Diablos's horn was broken so the other one grew to compensate, his rage was over flowing thus him killing many hunters, leaving their blood on his body. Because he swims and runs around in the sand, the blood's oxygen must've dissipated giving the blood stains a blue colour, thus cretaing Massacre King Diablos.

Just a theory though.