Mail-in votes should favour Biden, page-11

Alexandro, it is even worse for the Democrats which is why they are now trying to back track. The rejection rates of mail-in-votes when viewed by race are higher for black voters.

This is the observable effect but would not be a function of race in my humble opinion, it would more likely be tied to level of education. People unused to completing lengthy forms are more likely to make a mistake when doing so and in the case of mail in voting it makes their vote uncountable.

The point is, the Democrats rely on the black vote a lot more than do the Republicans and they simply can't afford too many black votes to be rejected.

https://www.propublica.org/article/in-north-carolina-black-voters-mail-in-ballots-much-more-likely-to-be-rejected-than-those-from-any-other-race