There's Literally Nothing Wrong With Having A Black Watson (Nor Holmes For That Matter) If That's What The Producers Want To Do
News flash assholes: Sherlock Holmes is a FICTIONAL character! π²
If the producers want to make a production to cater for a particular target audience - whether that be black people specifically or black people and knowing they will also pull a percentage of the white audience (whether that be that they genuinely don't care about casting skin colour or even if they do and see it as some positive, virtue signalling or whatever to watch) - they're totally free to do so. Their putting the own cash / career reputations on the table, not you, so watch it if you're happy or don't bother if it offends you so...
But really any argument against is as stupid as saying kids in a school in Africa, Tanzania let's say, shouldn't do a Sherlock Holmes play as they aren't white.
Imagine flying off to Tanzania and standing in that school's assembly hall, shouting your mouth off to any of the kids / staff who'd listen "You CANNOT do this Sherlock Holmes play because you are all BLACK and the Sherlock Holmes character as originally conceived by Sir Arthur Conan Doyle was WHITE!! So stop doing your play right now!!".
You'd look like a complete fuckw-t. And that's what you look like complaining about non consequential nonsense like this...