Besides what else has been said about it being actual blood, in Mosiah 3 King Benjamin said (perhaps someone already mentioned this?):
7 And lo, he shall suffer temptations, and pain of body, hunger, thirst, and fatigue, even more than man can suffer, except it be unto death; for behold, blood cometh from every pore, so great shall be his anguish for the wickedness and the abominations of his people.