Have you just had some below freezing temperatures? I would bet it's frost bite. If it is, those black parts will fall off. It won't permanently affect your rooster. If you see the weather calls for more freezes, you might try smoothing Vaseline on his comb. Also make sure his sleeping quarters are well-ventilated since it's the humidity build-up combined with the low temp that causes frostbite, rather than simply the cold.
Respiratory infectations can cause this too, but you should hear coughing or sneezing. The infection would be preventing him from breathing very well and creating a lack of oxygen in his blood. Another possible problem can be the heart. If his heart is giving out that will lead to a darken comb from the lack of blood flow. These are just 2 problems that can cause a blackened comb. More details are needed to narrow down the problem. Matt
Pics might help determine what is ailing your bird. Is the comb generally blackish, or are there black spots distributed on the comb?
