Well, in Henry the Green Engine, in the story ‘Henry’s Sneeze’, the story now ends with the boys running home “as black as soot”. I have in my possession a 1969 reprint of a 1967 edition, and the last word in that phrase is similar to the name of a State/River in Africa (Niger) but has an extra “g”.By the 60s, I think it was generally accepted that the “n-word” was offensive, but more than that, this was a clergyman writing, not some retired major born and bred into the British Indian Army...
Then there are the Scottish Twins, Donald and Douglas, portrayed as fond of a good sing-song but also clearly portrayed as heavy bruisers, seeing as they were built in Glasgow.