Well, Led Zep is certainly "blues-rock," and blues is certainly part of folk music, so according to some sort of logic, one might be forced to admit LZ to the category of "folk-rock."

To me, though, "folk-rock" denotes something a little less hard-edge than Zep, Cream, the Stones, or even Muddy Waters, Howling Wolf, or for that matter Paul Butterfield. To me, examples of folk-rock would be the Byrds, Lovin' Spoonful, Dylan and the Band, maybe even the Grateful Dead (though perhaps not their entire repertoire).