Yeah, it's a killer. I would definitely recommend treating as quickly as possible. I think Corid is actually amprolium. Which is what is in the medicated feed. Corid is at a higher concentrate though. There are other things you can use, but I think Corid is the least damaging to their system...
Usually blood in the stool in young chicks means coccidiosis. I think Corid is what you use to treat it with. Here's a link with an explanation of coccidiosis http://ultimatefowl.atwiki.com/page/Coccidiosis.
I ask a friend of mine, who is a nurse, to try and help me understand what exactly...