I'm really frustrated because I am in the same situation as the original poster. My coop has adequate ventilation, and my internal temp/humidity monitor indicates the indoor humidity is roughly between 40-50% on any given day. To me, that seems reasonable, as it is generally the same as or lower than external humidity (which obviously I can't control!). I do not have any frost or ice forming inside the coop. There is no water in the coop, and it is bed with very deep pine shavings and two straw bales inside along the front of the coop which is most exposed.
I have three hens with frostbite that is definitely in the black stages, not just on tips of combs but covering some portion of the comb itself. When the bad cold began, they were given vaseline on the combs as "prevention," but I don't feel that made any difference. In fact, I rubbed on so thick, that I think it matted down the short feathers around their comb and prevented them from puffing up those feathers for comb protection (which I saw my hens with pea combs doing). Once the vaseline rubbed combs became black, I applied Neosporin. This seemed to help with some temporary healing, but for 2 out of the 3, the black has not gone away (this is now about 2-3 weeks into it). When touched, the combs feel very dry and somewhat hard, though still pliable.
Two days ago I did a bit of an experiment. I have 2 Golden Comets with about the same amount of frostbite (though one has slightly longer tips left). I reapplied Neosporin to one Goldie, but not the other. The one WITH neosporin still has a pretty black comb. The one without has more red color returning. This was not what I was hoping to see.
I just don't know what else to do, but hate seeing them like this. They just can't get any relief from the cold, in or out of the coop, so I think this may be with them for awhile.