It's a virus, so is untreatable except for removal of any affected plants. Couple of things - "black death" in its early stages can look like a number of other hellebore ailments (Botrytis, Coniothyrium, or Phyllosticta) and really only affects H.x hybridus, not the species or other more complex hybrids.

While viruses are often vectored by insects, so far there is no direct correlation between aphids and hellebore black death.

Thanks for this info. I don't know which species I have as they were gifts from several different gardens. I am assuming that it is still a good idea to trash the infected plants, no matter what the cause.