Re my previous comment that Lomax might have had something to do with the political aspects of the 2nd revival, the book we are discussing quotes MacColl himself on the important effect that Lomax had on him.

Regarding the question of whether blues had 'any political influence', the book mentions the Ballad and Blues Clubs. I thought it was accepted that MacColl's contemporary, Lloyd, also saw blues songs as some sort of example of what folk music might be. This is in his book about Folk Song.

My own view, based on my own experiences and those of others, is that blues, both of the sort collected by the older Lomax and as it evolved via jazz, was part of what made many young British aware of the US colour bar, though this still shocked musicians who were fans of the blues when they went over there and encountered it. I would count this as having a political effect. Look at the work of Paul Oliver, for example.