It probably has to do with what was imported to their music stores. Records were fairly new in the 40's and 50's, especially good ones. And blues probably suited life in the UKIt's interesting that nearly every noteworthy British rock band at the time was heavily influenced by black blues but even those bands thought Zeppelin were regressive thieves. The Beatles didn't fukk with them, Rolling Stones didn't fukk with them, etc. And they were going on record straight up saying they didn't fukk with them band lol. There are multiple interviews of them getting shat on.
I can't speak on if they were racist or not. I have no idea. But I do like Zeppelin and it's hard to deny that they evolved their sound beyond blues later in their career and made great music.

