"Black comedy" is defined as...
...perverted and morbid...[depicting] situations normally thought of as tragic or grave as humorous.
Certainly Boo by Sally Clark can be seen as black comedy in that the jokes, while loosely fitting one's perception of funny, are pathetic and sad, and even sarcastic and cold.
For example, at one point Harry is having a discussion with his third wife Patsy. Her answers are nonsensical and annoying. Where he threw his second wife down the steps for this kind of talk, perhaps he is now too old or has mellowed enough not to use physical violence. Instead he insinuates that Patsy is mentally inept, asking if anyone ever "put a bullet through [her] brain." Patsy, who is rather clueless, misses the insult and says, "You're a kidder, Harry."
Moo behavior provides other examples of black humor. On her sixtieth birthday party with the entire family, Moo's boyfriend Wally has his hands all over her, while she giggles. Her grandniece asks her mother in wonder why the man is trying to pull her aunt's clothes off. The pair then move under the table to continue their carrying on. At one point Moo's sister Sarah and Sarah's daughter Jane are discussing Moo's behavior with Wally.
This is disgraceful. I asked you not to invite [Wally.]
Moo likes him.
That is apparent.
It's her birthday.
Must she celebrate here and now.
I don't think she's doing anything wrong.
Have you looked, Mother?
This is humorous in that Sarah excuses Moo's behavior, saying she is sure the two adults under the table aren't "doing anything wrong," but at the same time, she has no intention of looking—inferring that they probably are doing something wrong. However, this is sad in that Moo still acts like a child, even at sixty. And her sister Sarah is still just as vacuous as ever.
At the same party, Moo's childish behavior is exhibited again when she gets into an argument with Ditty about who painted pictures of trees.
Ditty, you dodo brain. I painted that tree and you know it.
You did not.
Would you two shut up! It's your birthday, Mooley. Remember that and behave yourself.
The bickering of the sisters shows them both to be extremely immature. Even Sarah's reprimand is that which someone would deliver to a child. Behavior in Moo that as a young woman might have been contributed to being indulged by parents or traits of free-spiritedness in a "modern woman" is now simply sad and embarrassing—or would be if the rest of the family wasn't so dysfunctional as well.
This kind of humor is not that which lifts one up, but that which displays the frailty of the human character—Moo's sad circumstances that can be traced back to a family of shallow and non-supportive members, and a life that lacks fulfillment. This kind of comedy makes one uncomfortable: though the laugh may come from "an element of surprise," it is not a satisfying one.