Your chickens have turned into VAMPIRES. Don't worry, all chickens love blood. Chickens figure out that the blood feathers will spout endless amounts of their favorite substance- blood and it's like their soda fountain. Blood feathers are basically the young feathers that grow after a molt-- then once the feather has finished, the blood supply goes away, but until then ...
I've got 2 separated for this very reason. Because blood is most visible on light colored chickens, they seem to get the worst of it because the red is like a target- so for me, even with 20% protein and huge living spaces, I just take them out of the flock - until the feathers grow back the rest of the way. I've got pens adjacent to my main run so they stay socially acquainted but can't be harmed.