There are various stages of frost bite. I wouldn't think a few nights in the 20's would do it. Minor frost bite/frost nip, can heal. If the tissue was frozen solid through to the core, as in ice cube, that is a problem that will cause the loss of a tip instead of just peeling of the outer layers. Large combs can turn purple/black if they lack circulation and are cold. Older birds like older people can have reduced circulation so can be more prone to frost bite. If you press on the comb part that is black, does it change color? If it blanches to a white and turns purple/maroon/black again, it might just be very cold. If it's black, hard and/or schriveling though, that's dead.
