One of the things that always impresses me in a drama or tragedy story is the injection of humor. This is a remarkably difficult mixture to achieve, and this is all the more reason to value it.
You may well ask: “Why look for humor in non-humorous works? That’s why they have comedies.” Well, this is undoubtedly true—although the comedies I like are typically darkish comedies—but I have noticed that a really effective serious, tragic story often requires a dash of wit to make it believable.
Consider, for example, the film which I consider the best I’ve ever seen, Lawrence of Arabia. It has a rather depressing ending, and yet the penultimate scene ends on a line from Claude Rains to Alec Guinness that I always laugh at—and while it is a sardonic line, I can’t help thinking that most screenwriters would not have had the courage to put it something so drily funny in at such a moment. Yet, it works.
Another favorite film of mine is Chinatown, which has one of the darkest endings ever. But it also has some very witty lines an amusing scenes early on. The writer for that film, Robert Towne, described it as a“tunnel at the end of the light.”
I don’t deny that this mixture is extremely difficult to pull off. This is all the more important because trying to do it and failing leaves a much worse taste in the audience’s mouth than steering clear of it all together.
The movie Star Wars: Episode III: Revenge of the Sith (keep in mind that this is my favorite of all six Star Wars films)  fell into this trap a bit in the first third of the film. I suspect George Lucas was aiming to provide a “tunnel at end of light” effect, but what he ended up with was a lot of comic business with R2-D2 that didn’t fit with the dark tone of the film. (This is Lucas’ biggest flaw as writer in my opinion—he cannot do wit, he can only do jokes.)
This same concept of mixing humor with a grim overall story seems to apply in other media as well. It’s one of many, many reasons I adore the video games of Chris Avellone, which usually contain both considerable wit and  deep, dark philosophy, impeccably mixed together.
Of course, there is an alternative way of achieving this mixture: the black comedy. I tend to feel that dark subject matter lends an “edge” to humor, although the difficulty is that one man’s dark humor is another man’s appalling perversity.
That’s the danger with dark humor, that the converse—what I’ll call humorous darkness, awkward though the term is—does not have.  Inject too much humor in to something that is at its core serious and people will just not take it seriously. Inject too much darkness into something that is supposed to be a comedy and people will think you’re sick. 

What's your stake in this, cowboy?