Black may not have a permanent position (Mitchell, Wiemer, Frelick and Chourio are all better defensively in the OF) right now but he isn't as bad as Keston was/is defensively. He may not have the arm to stay at 3B but range wise and glove wise he is fine there. The same for 2B and even CF though Mitchell and Wiemer are far better defensively in CF than Black is.
I think Black gets a bit of a downgrade unfairly because of his defense. He may not be the greatest at 3B, 2B or CF but he isn't a liability either. I think he is just average maybe a bit below or above average depending on which position you put him at. The problem is that Black is behind Turang better defensively at 2B and behind Mitchell, Wiemer, Frelick and Chourio in the OF. The best defensive position for Black is also probably his worst right now.
This could all change in the off season with a trade of Adames where Turang then goes to SS. Black then has a defined position where there isn't another prospect or player blocking him. Maybe Urias would block him at 2B but you could always just have Urias at 3B assuming he isn't traded also.