I would advise you to stop spending so much time looking at her stools. :)
This is a fairly common question. As long as the calf is bright and alert and active and no significant diarrhea, I would ignore it.
As to why there is some blood, sometimes there can be some inflammation in the intestines and some blood will pass. Inflammation possibly from undigested milk or milk replacer going to the lower intestines and fermenting.
And possibly some constipation or eating roughage which might make the stool more abrasive when passed. Sometimes with constipation you might also notice some mucus (slime) on the stool. If I recall correctly, the rectum has mucus secreting cells in it's lining for lubrication, and if a stool takes longer to pass, the mucus builds up on it.
Or so I've read. Someone may correct me or add to what I've said.
Is this callf on a cow or a bottle?
