The thing about blues is that there is a common identifiable tradition here - there could be arguments about the boundaries and whether something can properly be classified as blues, and about White Blues, and Country Blues and where it shades into Jazz and so forth but I think there'd be a general agreement that all blues had to have their roots in a particular tradition.

And you can't do that with folk really, for the roots of what gets called folk are all over the place, in all minds of unrelated traditions. Chinese music, Irish music, Indian music, Romanian music, English Music Hall.The list goes on for ever...

I suppose the next logical move in the discussion would be "Are all blues folk?" And if some blues aren't folk would that mean they aren't really blues? (My answer to both questions would be No, for wehat it's worth.)