Sounds like myco I had a bout of it go through my breeders when I went out of town for a few days and my sister forgot to clean them. If it is myco, it's not blood, it's porphyrin stains. I ended up getting rid of the rats that had it the worst, but got my pet male through it. He still gets "episodes" of it now and then, and it will most likely be what kills him, but so far he's doing pretty good.
http://www.rmca.org/Articles/myco.htm
Also, the site lists some antibiotics to use. I didn't end up using any, just cut down the number of rats in each group, and started cleaning cages at least every other day. Good luck