You might have bought a few of these black items without knowing their purpose. The description for these items read “used to transmutate,” they don’t become useful until August 1st when you unlock the electric chair execution.
Once a day, you can fuse certain personas using the electric chair into an accessory (black rock), a weapon (black kogatana), a gun (model gun) or armor (black robe). The strongest weapons in Persona 5 are obtained through item fusion using the strongest personas.
These items can be purchased from the Dark Net (run by Tanaka from P3&4) once you purchase and fix the laptop from Akihabara. The black rock is available immediately, the black robe is unlocked after spending 10,000 yen, the black kogatana at 50,000 yen and the model gun at 100,000 yen. Alternatively you’ll also be able to find them for purchase around town. Black rocks can be purchased from the peddler in Shibuya at night. Black kogatana can be purchased from the used goods shop in Yogenjaya, same place you bought your TV. Model guns can be traded to the boy next to the capsule machines in Akihabara if you get a Mothman figurines out of the capsule.