Learn German: Der Humor ist sehr schwarz, worüber sich die Leute amüsieren. - The humor is very dark, which the people are amused by.

worüber is acting as a continuative relative pronoun here. Instead of pointing back to a single noun, it points back to the entire previous idea: the fact that the humor is very dark. When referring back to a whole clause or concept using a verb that requires a preposition, German uses wo-compounds like worüber.

Where does the über part in worüber come from?

It is dictated by the verb in the relative clause: sich amüsieren über (to be amused by / to make fun of). Since the people are amused by the situation, the required preposition über combines with wo- to connect the two clauses.

Why does the reflexive pronoun sich come before the subject die Leute?

In German subordinate clauses, pronoun objects like sich prefer to jump as close to the beginning of the clause as possible. They usually land right after the connecting word (here, worüber), which pushes full noun subjects like die Leute to the right. If the subject were also a pronoun like sie, it would stay in front of sich.

The English translation says dark, but the German uses schwarz (black). Is that normal?