Is the following sentence compound or complex? “He blushes; therefore he is guilty.”

If ‘therefore’ is removed, it will not become compound because it is a compound sentence.
A compound sentence is connecting two sentences using a conjunction, where two sentences can stand on its own.
If you remove ‘therefore’, you have two separate sentences.
In a complex sentence, you have a main clause and a subordinate or dependent clause. The main clause will stand on its own whereas, the subordinate clause will not make complete sense.