If one were to describe a statement by referring to "black humour", how should he/she go about forming the adjectival form of the term?
Blackly humorous is the form you want. (One job of an adverb ["blackly"] is to modify an adjective ["humorous"].)
Example from a book review: “A blackly humorous fantasy full of monsters, demons, and witches, there’s more than enough here to engage the reader looking for that, but beneath the surface The Sad Tale of the Brothers Grossbart is a novel of surprising complexity.” —Realms of Fantasy
I've seen blackly humorous before (without the hyphen), so I'd plump with this. It still looks very unnatural, though.
Why don't you use sardonic as an adjective?