Aibou wa Slime!? ~Saikyou no Aibou o Eta Boku ga Saikyou no Mahou o 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)