Technically, blood in feces can be from cocci, bacterial enteritis, or other causes. However, most often it's from cocci - especialy in babies, and almost always in babies on non-medicated food. Cocci are difficult to diagnose, even in a vet lab, so we treat based on that symptom. Many people use Corid because it's gentle on gut bacteria and deadly to the protazoal cocci that cause coccidiosis. In older birds, I use Sulmet or any time I highly suspect E. coli in younger birds.
In your case, I'd treat with Corid or Sulmet in the morning. Tonight I'd start her on a probiotic and continue that throughout her treatment. Then after treatment, you can give every other day for a couple of weeks.
My rule of thumb is that for every day I treat, I give probiotics for twice that many days afterwards but only evvery other day.
7 days of antibiotics = 14 days of probiotics given every other day.
Just a note about organic feeds. They replace the coccidiostatic activity of amprolium of medicated feed by using probiotics, live bacteria, in their feed design. This is great - however - live bacteria don't necessarily always live long in a bagged feed on a shelf in the conditions that we or the stores keep feed. They do better, stay alive longer, in cool conditions. So you won't be getting as much power from the probiotics in the feed as you might want.
If you choose to do organic, you really should go ahead and give all the babies additional separate probiotics at least weekly for the first six weeks. What you're doing is making SURE that the cocci are held at bay by good bacteria. Medicated feeds are fed for six weeks, but you're replacing that with probiotics. Then if you see any problems, use Corid (in healthy birds).
In your case, I'd separate the sick bird and a buddy. (They do better with friends.) Treat them with Corid and then give them probiotics as directed above. The rest of the healthy babies I'd simply give probiotics every other day as directed above.
Clean and disinfect all the feeders/waterers, and replace the bedding once you've removed your ill baby and his friend to the other brooder.
Probiotic choices: Probios or fastrack from the feedstore, or ask their associates to show you their live-bacteria probiotics in the horse/cattle aisle. Make sure that they at least contain live bacteria (usually shown in CFU's, culture forming units) and not just byproducts of bacteria.
TSC has probios in powder and paste. I prefer the powder, and it lasts for a long while in the fridge - and stays alive. You simply woulld sprinkle it on their food at the first feeding in the morning. Or you can mix it into a quickly eaten more wet treat. (Don't use in their water - for any probiotic). You could also buy the paste if it's more economical and use a little dab for each chick.
Other options: plain yogurt, live culture (dannon, yoplait, etc). 1`heaping teaspoon usually treats 6 chicks.
Acidophilus capsules or tablets (grocery store, health food store, pharmacy) Crush tablet or break capsule onto feed or into a treat.
I hope this helps!
