There is an expression that has become proverbial in English: “Some of my best friends.” People who start off by declaring, say, that some of their best friends are Jewish, usually come to a “but” or “however,” followed by an anti-Semitic tirade. In the 1970s there was even a play about anti-Semitism called Some of My Best Friends (running in New York). Those who begin with this expression are immediately labeled as anti-Semites—and in fact some time ago, paradoxically, I decided that the best way to begin any discourse against racism would be to start off with: “Some of my best friends are anti-Semites…”
Umberto Eco. Turning Back the Clock
Stuff Jeff Reads 
Some of my best friends – is also used in relation to Black people in the UK. A phrase I grew up hearing a lot. Another phrase was “You’re alright it’s the other’s” . Both these phrases disappeared until the 20 teens. Now I frequently hear or read about so and so (and occasionally directly to my face) “They/you have a chip on your shoulder”. Old phrases, same racism and beliefs.
Hi DQ. You’re absolutely right. It is a qualifying phrase that is almost always followed by a racist or derogatory comment, regardless of the group it is referencing. It’s like, fill in the blank: Some of my best friends are _____ (gay, black, Muslim, liberal, conservative, etc.). I will add that while those phrases may have “disappeared” for a while, clearly the sentiment underlying them remained beneath the surface. Hopefully, since it has been exposed again, we can begin to address the core issues. Hope you are well. Blessings.