I've been on the outside of, but regularly exposed to, police humour for more than 25 years.
Describing it as 'black' is inadequate, it is far more complex than that.
Police humour is variable and unpredictable, what you might think would amuse them sometimes doesn't, and can even cause offence.
They may also find humour in remarks which most people would say there is none.
As a body of men and women, coppers are a strange bunch all round.
I've done stories which I think will cause ructions, but cause no reaction, yet another story which to me is innocuous can cause chief constable level ructions.
I think it's something to do with an 'us against the world' attitude, but short of doing a doctorate level study of the topic, I can't take it much further.