Aibou wa Slime!?: Saikyou no Aibou wo Eta Boku ga Saikyou no Mahou wo Tsukatte Nariagaru

Zack, the protagonist, has hit rock bottom. His family hates him, and his fiancée has called off their engagement. A black object appears before him. It turns out to be Sura, a powerful black slime capable of human speech! A great adventure, with the strongest slime! They're aiming to rule the continent!? (Source: Gentosha Comics, translated)