It's the second definition listed on wiktionary, which is usually quite reliable; that site doesn't list its second definition as obscure, archaic, or obsolete; OED agrees that the derogatory connotation is secondary but operational/current
I'm not denying that people out in the world have used the term derisively; the point is that such usage is waaay less common, to the point that it is almost unknown to most people who use the original meaning. And it's not that it doesn't exist in any dictionary anywhere, but that it is uncommon enough that many do not include it.
The secondary meaning almost certainly comes from the fact that it
sounds like a racial slur, which becomes somewhat self-fulfilling (this is what AA's quote ended up saying).
Personally—and maybe I've just read too much Toni Morrison, alongside none of the Br'er Rabbit stories—I was actually unaware until now that "tar baby" had any connotations that weren't overtly racist; I've always seen (or heard) it used, essentially, synonymously with the term "pickaninny".
I don't know how you could have read Toni Morrison's
Tar Baby and not gotten the "sticky problem" meaning. Morrison is hardly known for the subtleties of her metaphors, especially the ones in the dang title. I haven't read that one, but I just can't imagine she doesn't explain the story and the moral.
I have ask, just how often are you hearing people call black kids "tar babies"? I've literally never heard that in my own personal life (though I accept that it surely happens and believe AA when he says it happened to him). Seems weird to me that you would have heard this so often as to have a strong memory of it, but never heard the original meaning. In contemporary news, it mostly just comes up when old white politicians get yelled at for using it in the non-racial way.
I think it's somewhat of a different species than "niggardly", which has absolutely zero connection to that other word outside of slant homophony: not only is the meaning of a substantially different sphere, but the etymology (likely, ultimately, Old Norse) is world's away from our nation's history of chattel slavery and racial subjugation.
Gotta disagree here; I think your own anecdote shows how it mirrors niggardly, even if not perfectly exact. You said were personally unaware of the original completely non-racist meaning of the term
until just now, and have been assuming it is an insult (I am guessing based on how it sounded combined with scant personal exposure; anyone with a full familiarity with the term would know the original meaning). I also think my comparison to the word "monkey" still holds, perhaps more accurately. The main meaning is non-racial, but calling a person such a thing is an obvious racial slur.
The phrase "tar baby", though—even with a separate, standardized meaning, which was/is perhaps not meant to be racially-charged—nonetheless derives from stories about the American South and subjugated African-Americans.
I not sure how this makes the origin of the term "racist" in any facet. It's not like calling the Big Dipper the Drinking Gourd is somehow a racial epithet.