the black parts you're seeing are basically necrotic tissue (tissue that has died due to the injury inflicted upon it).
the injured rats are going to need antibiotics to fend off infection and/or gangrene setting in to the wound. the injured tail worries me most of all, because it is actually a very important part of the rat. you need to get that treated right away so infection doesn't set in.
you need to take them all to the vet as soon as possible.
and take out the rat that bit them before he/she causes further injury. if that rat seems to get along with another rat or two (and doesn't bite them for whatever reason), put him/her in a different cage with the rats it gets along with.