In my experience, small amounts of occasional comb-darkening are normal. Especially at the back (blade) of the comb. My birds with larger combs will sometimes get this after exerting themselves, or if they are very excited and "flushing". Purple tints to the comb have to do with bloodflow and oxygen levels in the blood, so it makes sense that on occasion when their bodies are using more oxygen in other areas, the comb (which, to a chicken, is much less vital than other parts of the body) loses some of the bloodflow first. Was your bird previously very excited? Simple things can be exciting to a chicken, such as feeding time, or the appearance of a "Food Human", or activity by other birds. For roosters, other things may be exciting, as well! He is at that age.
If more of the comb turns purpleish, for longer periods of time and/or more frequently, that would be a cause for concern. Observe this guy and let us know what you notice! Check his breathing as well. If his comb goes overall more purple, or does it more frequently, be sure to observe him closely.
