It's because of his sister getting blacked
For two reasons

One of course
>There's this educated gentlemanly black British Army WW1 veteran
But more jarringly
>Tommy is fiercely protective of his sister and hates every love interest she ever had UNTIL the black guy, who he immediately likes and respects
It's even more jarring in the context of the series just for the way he threatened or insulted every previous partner

Also there is a single bizarre scene where the crazy old aunt rants to the sister about the genetic superiority of mulatto children. I'm not shitposting but the context is that she's an eccentric who thinks she talks to ghosts