That can be a symptom of coccidiosis, are there other symptoms, other ways this chick is acting ill? Lethargy, lack of appetite, puffed feathers/fluff? When I find bloody poops from my chicks I give them a liquid medicine called Sulmet. It should be available at your feed store. There are directions on the bottle, you give a certain amount for 2 days, then half that strength for 4 more days. You put it in their drinking water. I'd give it to all the chicks, let them all drink medicated water. Once that sick chick begins to feel/act better you can put her back with her friends.
Get well soon, chickie!