Does my rooster have frostbite or is it something else?

I have heard that putting petroleum jelly on combs and wattles helps prevent frostbite, but I do not believe it. If the black spots are from frost bite, leave them alone but watch them for signs of infection. If you pick them off, the tissue beneath it is more vulnerable to frostbite. Both of my large roosters develop black spots on their combs during the winter. they are never very big and they heal in the spring.

Of course they could also be pecking wounds. My roos get those when they scrap with each other too.

 

It is probably frostbite, but black wart-like spots may be something else too:
https://www.backyardchickens.com/web/viewblog.php?id=31743-fowl-pox-cocci-mareks-info

With your temps that low, unless they look warty, I would guess frostbite.

 

Could also be fowl pox, there is a dry and wet version and the dry has black flaky spots on comb and wattle. It can pass through a flock quickly they say and takes about 30 to 50 days on the outside for it to run its course. It is commonly caught from wild birds. The dry form is not fatal and afterward your flock/infected birds will be immune.