A dark comedy usually involves making people laugh about things normally not designed or thought to be funny. An example might be a killer that makes funny remarks after brutally killing people.
Another example is the movie "War of the Roses" about a couple going through a bitter divorce while living in the same house and physically out to hurt one another. "Bad Santa" is another dark comedy. An alcoholic Santa who cases malls to rob them.
You have to draw the reader or viewer "into that world" where they suspend their own moral compass and become willing to follow the moral code of the characters involved in your story. This allows them to find humor in things they'd ordinarily would not consider to be humorous topics. The more over the top it is the better.