Yes and no.It really wouldn't be a disaster with that number. As I've said before, the numbers can tell only one story. If you're looking at a poll that shows Kamala +3, and she's only at 80% among black people . . . it means she has support elsewhere. For that to be bad for her, you'd have to assume the 80% holds but the stronger support elsewhere doesn't.
Remember also that composition effects matter. Kamala could do worse with every single demographic group than HRC and still win, if the mix of demographics changes. If black people were 10% of the electorate in 2016 and 15% this year, then the black vote will be better for Kamala even if she's not winning as high a % of it.
Voting constituencies do change over time -- e.g., the non-educated white voter outside of the south switching from democrat to republican after Obama. But these changes tend to happen very slowly, not all at once. The black vote has been solidly democratic -- roughly 90%, give or take a few points here or there, especially when Perot ran and could steal a few votes. If that number dipped all the way down to 80%, I don't believe there would be enough new constituencies to take up the slack, especially in the swing states of Georgia, Michigan and Pennsylvania.
That said, as I wrote above, I don't believe the final exit poll numbers will be anywhere close to 80%. When all is said and done, I expect those numbers to be between 85% and 90%, likely closer to 90%.