I worked with someone years ago who would get annoyed if you described anyone by the colour of their skin - i.e. if he asked who so and so was and you said 'oh it's the black guy over there' he'd find it offensive - essentially he didn't like being reduced to the colour of his skin (had grown up in Montreal and often felt that 'black guy' was a way of indicating he was out of the norm or different merely based on the colour of his skin.) So using his logic I get how 'black professor' could be deemed racist in that it suggests that a black professor is out of the norm. It isn't a homogeneous definition of course, I remember going round the houses one day to explain who somebody was to another co-worker, he heard me mid way through and just piped up with 'I'm the black guy over here!'
I've always seen it as context heavy.
But sometimes it's worth noting someone's appearance or background. I mean Kevin Campbell was Everton's first black captain. Likewise, if an intruder was say 5 ft 11, white skin, brown eyes etc. I don't know, and it's probably not for me to say but I think at times we are focusing on a lot of the wrong things with the stuff around race currently.
If people get offended then I wouldn't say it, because I suppose all other codes I try not to act like an idiot if I can and wouldn't want to upset someone to prove a point. However if someone referred to me as a white man, I can't say I'd be offended as it's a descriptor.
There's lots of stuff happening to me thats serious, knife crimes, poverty, black people who've lived their lives here being sent to other countries because they can't produce a bit of paper by the Home Office yet the civil servants and politicians who have the power to resolve those issues get off scott free. Meanwhile ordinary black and white people are increasingly censored for what appears to me to be trivial offences. I can understand why a section of people just throw their cards in and say it's ridiculous.
As always I appreciate it's not the most politically correct response, and if people are offended by it I apologise, but they are increasingly my thoughts on where a lot of these discussions are heading.