What could cause blood in a newborns stool?

GI bleeding. Blood in the stool generally means that the GI tract is bleeding. If it's red blood (as opposed to black, tarry stools) then this suggests a lower GI bleed. You should immediately report what you are seeing to your pediatrician as soon as possible so your baby can be checked immediately today. If your doctor is not available take your baby to the er.
Blood in baby's poop. There are a variety of causes. If he/she is breastfeeding and mom has cracked nipples, baby can swallow blood and pass it through. If he/she is having constipation or bad diarrhea, the anus can end up with a small tear that can bleed. Very rarely, a baby can have a food allergy to milk or soy. More rarely, a baby can have meckel's diverticulum - a side passage off the intestine. See your doctor!
Blood in stool. Blood in stool in newborn could be caused from localized causes like anal fissure to generalized causes like maternal blood, stomache ulcer to milk protein allergy. You need to see your doctor for sure to rule out serious condition.