“Black” now depends as much on political correctness as on melanin content.
Clarence Thomas is not “authentically black” because he does not tow the Progessive line.
Elizabeth Warren was authentically Native American because she was politically correct.
As an aside, the phrase is "toe the line":
http://grammarist.com/usage/toe-the-line/
More detailed:
https://en.wikipedia.org/wiki/Toe_the_line
I'm only saying this, not to embarrass or offend you, but because I've seen this phrase misspelled so many times now on FR that I can't resist the temptation to correct it. If this were in a story you were writing for publication, the editor would correct it instantly.
If you think the spoken phrase suggests pulling a burden, a boat, along behind with a rope, that is not what was intended as an idiomatic expression.
But as far as Amy the Professette's contention is concerned, she may have been right statistically (properly qualified as to "blackness"), but injudiciously blunt in the current academic atmosphere.