The poster in the background is for a British TV program, The Black and White Minstrel Show. It did have a minstrel group. It makes me wonder if maybe Arnold made a guest appearance on the show? This photo has been nicely colorized:
I think that was a poster for the B&W Minstrel theatrical revue that toured the UK - not the actual BBC television show.
If Arnie appeared on the TV show, it would've come out by now , given the show's retrospective notoriety and how popular it was back in the day.