TheDevilsCorpse is currently releasing tweets about the newest chapter and it reveals more things about Black's power. When he first got Goku's body, he assumed he'd be able to use all of the power immediately but like Ginyu before him, the mind switch disrupted all of the progress Goku made since his mind is out of sync with the body (ki is a physhical and mental thing so this isn't too far fetched).
Originally, he couldn't even use Super Saiyan which explains why he doesn't kill Trunks, this also explains (with a later mention by Future Zamasu) that Black intentionally went to this timeline to power himself up so he could get stronger and move on to other timelines to continue the Zero Mortals Plan.
Every time Black gets hurt then heals, his mind and body become more in sync with one another which allows him to gradually activate dormant abilities like the Super Saiyan forms.