If you have a couple of birds with broken and bloody tail feathers- someone is probably pecking at them. Chickens are attracted to red/blood and will peck at wounds. Feathers when they are first growing in have a blood supply and may be pecked by a bored bird, a bird that has developed a bad habit, ect. Pecking and hurting each other can be a sign of stress (not enough space, poor nutrition ect).
The rooster's (and hen's) comb if injured will get a black scab. A pecked/bruised comb might look like what you are seeing. A photo might help.
What are you feeding and how much square feet per bird do you have, ie how big is the pen, and how many birds in it?
If the comb is turning purple and the rooster is acting sick (lethargic, getting skinny, eating less, sitting down, less crowing, mating ect) , then I would worry more about the color change to the tip. It is a very vascular piece of skin, so if there is something wrong with the circulation, the color can change pretty dramatically.
jess