If you instituted a policy to require less evidence to convict black defendants, you would convict more black defendants, which would make the measured "base rates for violence among blacks" go up, which would mean that you could need even less evidence to convict, which...
No, you'd just need to keep track of how often demographic considerations influenced the outcome, so that any measure of "base rates for violence among blacks" you use for such decisions is independent of the policy.
(That's not to say that such a policy would be a good idea of course)
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