Normally. If the blood is literally looking like blood then it's blood and there's a big problem , but when aros eat red krills w/o all the external parts(heads,tails,legs,shells) then they would normally release red poops, if fed with live foods, they release black poops, if pellets, they release brown poops. I also experienced the same thing but after my experiences with the poops of my aro that's the time that I learned that the poops color depends on what the fish eats.