plaster
If you met most of my family members, you would not think they were white, but if you met me personally, you probably would.
Only once in a long while does someone seem to read me as 'not white' (and unless I'm speaking my heritage language they usually guess wrong about what flavor.)
> were on a phone interview with someone you couldn't see, and afterwards, you said something like, "I was trying to determine if they were Black."
If I'm on the phone with someone, I am sure I am trying to figure that out. Folks who claim otherwise are most likely incorrect about what they are doing.
I would certainly know better than to say that aloud, since I grew up in a society where I know how it would be perceived, and have intuition around it. I don't have to think much to realize the implications.
I think you did the right thing in this situation. She did not express anything malicious and she corrected herself (at least partially).
It sounds like your colleague was doing something anyone would do (assessing someone by the social strictures they are familiar with) and did something wrong (devoted effort to it in a hiring context, said something aloud.) Yes, everyone does this sort of thing -- in American society overall we don't have good names for the subtle side of our class system, so we can't express the puzzle as clearly as some other societies, but we do see the world (consciously or unconsciously) along these lines.
When I hear people say inappropriate things -- usually less fraught ones like marital status -- I point out that they cannot say such things. It serves as a reminder of proper procedure. I do not escalate the situation if they seem to be acting without malice and, like the person in your story, seem to desire to do better.
(Full disclosure, I am a white-presenting man.)