To me you have just demonstrated one of the problems. The programme was MOCKING characters like this and not promoting that behaviour, AG was a well known lefty. People see something about race and assume it is racist. I have seen other people say that Ricky Gervais is racist cause of his David...
As a lefty I also really do not get cultural appropriation, to me it actually goes against inclusivity and multi-culturalism. Surely the beauty of culture is being able to share and adopt. Cultural appropriation seems to take something and say only one group or race can use it and no one else...
Had it been a one off fancy dress I would agree. However, the fact he blackfaced on more than one occasions makes it a little more difficult to defend.
What we do need to stop doing in society however is judging the past based on the standards of today. If we did, everyone was a racist...