It doesn't mean anything without specifying the denominator, they could've meant per unit time, population, or crime.

Now that you've specified though, the only remaining wiggle room is 'who is actually committing the crimes'.

80% arrests being black people is 'fine' if 80% of 'arrestable crime'[^] is committed by the black population; it just points to something other than policing needing improvement. Education perhaps, or welfare.

[^] by this I just mean that not all convicted crime involves an arrest, and maybe there are demographic differences in the types of crimes committed