I only hand milk but blood in milk can be from udder getting too tight, milker being too strong of pressure, low calcium and of course mastitis. I would start by testing for mastitis and giving 30 cc of a carrot and celery juice or other calcium source. I like the C&C juice as its a gentle calcium source that won't hurt even if calcium is not needed. Only hand milk until you settle things. Check everyone milk separately to know who it is.