Clearly Black Widow as a red head is an incredibly distinguishing characteristic for her, which is why she has blonde hair.
We have intentionally tried to strip that accent away for a couple of reasons.
Cap, we’ve seen Chris Evans walk around in public with a beard and no one recognizes him.
One is you’ll notice at the beginning of Civil War that Black Widow is training her to be a spy, and two is she’s been on the run, and one of the most distinguishing characteristics that she has is her accent.