Other than the dubious history of white people blacking up to mock black people, nothing. Same as there is nothing technically wrong with the word spas, as it comes from the word spasmodic, relating to the uncontrolled gestures of cerebral palsy. However, you cannot simply disconnect from the context that blacking up entails.

I think it does tend to get a pass when you go as Someone specific. Friend of mine went as Mr T to a stag do, and the staff (black) at the After Dark absolutely loved it, and it has a kind of necessity.

But when its 'generic sunglass seller' there are overtones that it probably would be wiser to avoid, given the history of offence and controversy around blacking up.

"One Nation Under God, has turned into One Nation Under the Influence of One Drug"
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Be careful what you wish for Jamie.

I think that the thought police/fascist liberal agenda has enough power already. The know no limits in their quest for appearing virtuous and the runaway effect fueled by profit making concerns is out of control.

We are teaching people that being easily offended is the way forward. It is in fact, the way backward.

We all need to start taking ourselves less seriously.
The problem with this stuff is that is almost always politically, media or commerce driven. It is all about self serving agendas and little to do with actual offense.

We will never go back to the 60's and 70's. People are now used to seeing lots of non white faces.

This is about how we want to be as human beings. Do we want to all run around acting like school children taking offense where ever possible and finding new ever more feeble reasons to dislike each other or do we all adopt a more mature approach and stop pandering to pettiness.
How offensive is blacking up in the grand scheme of things? Is it actually worse than whiteing up? Answer, no. To continue to constantly reference historical examples of prejudice to justify saying the opposite is counter productive. This is the here and now and we all need to grow up and recognise that this sort of respect for others feelings has to work both ways and that tolerance sometimes means giving people of all colours a little wriggle room for taste and manners. What ever you say and do you will offend someone. This must be recognised sooner rather than late in a world where opinion travels round social media like a pandemic.

On what basis "take issue"? As an unaffected person presuming to know what is offensive to others in a patronising misunderstanding, or because they are a sunglasses seller that has been persecuted based on their ethnicity?

Before anyone thinks I am being racist, I don't buy them because I need special prescription sunglasses what with being as blind as a bat, not because I wouldn't purchase knock off goods from a fella based on his ethnic grouping.

Only if you push a black fella off a deckchair to get a prime spot.