I think that is a bit of an exaggeration. Yes, there is some casual racism, but nothing untoward for the time. As far as I can recall, it rarely comes up, because non-white characters (with the exception of Gypsies) are rarely encountered; and that was no more than an accurate portrayal of how things were in England in that era. (Well, again, except for the implausible overabundance of Gypsies, but IIRC [perhaps I don’t], the Gypsies are by no means uniformly negatively portrayed.)
Of course there is the Golliwog in the Noddy books, but although golliwogs were no doubt racist in their original conception (whenever that was), I rather think that in 1950s England that had been largely forgotten, and they were just considered to be traditional children’s toys, like teddy bears (which have surely never made any kid think about Theodore Roosevelt and the Bull Moose Party). My parents were firmly anti-racist, somewhat politically correct ahead of their time, but I had a cuddly golliwog toy, and can remember my mother helping me to collect jam jar labels to get little golliwog badges (the jam company had a golliwog as its trademark). It never even entered my head that that golliwogs were meant to be representations (let alone offensive representations) of black people. Blyton, as an adult, may have been aware of the fact, but i doubt that it was in the forefront of her mind in creating the Golliwog character: Noddy lived in Toyland, a land where all sorts of vaguely anthropomorphic toys were alive, and could talk, and cuddly golliwogs were standard - probably almost universal - inhabitants of a 1950s British child’s toy cupboard.
It occurs to me that younger people, these days, may not even know what a golliwog looks like. Here is a picture; oh, and here is one of those badges, from the jam company, that I was talking about. [Warning: Politically incorrect links!]