If they are doing that again this time, that could affect some Senate and gubernatorial races.
But it's not likely to affect House races much, since African-Americans tend to be concentrated in House districts that are already held by the Democrats, districts Republicans won't win.
If the polls are wrong this time it would probably be because they overestimate opposition to Trump by suburban voters.
If they don't, they may be on target this time.
Off year elections are very difficult to predict, and can vary wildly from district to district and state to state.
Weather is a much bigger factor in off year elections than it is in presidential years, and that is just one example.
The media’s biggest lie is that they have a clue what turnout will be—they don’t and nobody else does either.