I have never heard an accent like the one she feigns.
She might not be a good mimic. She grew up with her parents’ accents for the first few years of her life, then her home accent was probably modified by what she heard on TV and at school in a natural way. Her HBCU time may have contributed the most to what she affects. It’s just not a natural accent for her. She’s just drawling out her words (aaargh, in her somewhat chalkboard-scraping voice) to sound “black”. I never said it was authentic, because it can’t be
My father’s father was straight from Italy and had a strong accent despite being in the US since the late 1920s, but all of his children that I met had a Rhode Island/NYC accent.