Ooh, an excellent question, because of course it does quite deliberately set out to create humour, so you're hitting nicely on how the two genres relate. And I certainly think a bit of macabre humour can fit nicely into a horror film - e.g. Miles Malleson drumming on the coffin lid in Dracula or a crisp "Pass the marmalade" in Curse of Frankenstein. The emotional relief which those scenes create can help to keep the texture of the film varied, and stop its more serious scenes from tipping too far over into unrelenting grimness or (worse) melodrama.

It's perhaps a question of which is spliced with which. Theatre of Blood delivers real horror spliced with black humour, but The Comedy of Terrors seems to me primarily a farcical run-around which happens to be set in a firm of undertakers. Certainly, you could show a child The Comedy of Terrors without too much worry, because although it deals with death and murder it isn't very realist, but you'd be best advised not to do the same with Theatre of Blood.