Why is Black Widow suddenly a redhead again? Why make her blonde for just one movie?

She probably went blonde to hide her identity a bit since she was technically a fugitive after Civil War. That being said there's a rumour that quite a bit of time has passed since Infinity War so her natural hair color grew back out. To me it looked like there was still some blonde in the tips so it's not completely out yet.

Black Widow, has been on the run and undercover with a new identity and look.

At the end of "Captain America: Civil War," Tony Stark tells her that T'Challa/Black Panther told US Secretary of State Thaddeus Ross that Black Widow helped Captain America make an escape from the government.

So it should be of little surprise that Black Widow is rocking some new locks.

Romanoff never addresses her look directly — and she doesn't need to. Captain America doesn't explain his new beard in the movie and he's also on the run. It's understood when Rhodey has a scene with Ross where he's told to arrest them when they show up to meet with him.