Blacks are what, 13% of the population? So if you increase your support there by 10%, you gain only 1.3% of the popular vote, but the Dems would also lose 1.3%, so that’s a 2.6% shift, which would be pretty significant, enough to decide a tight race.
The recent support has been at around 5%, so there is room to move the needle by quite a bit more than 10%.