So, were Led Zeppelin a blues band, at least to begin with? Or were they the thin end of the wedge, leading to and responsible for the ubiquitous, bland 'rock' which serves as the wallpaper for North American radio stations? How much of the blues can you still hear in 'rock'?

Led Zeppelin were definitely an "acid blues" band. Granted, they paved the way and inspired many other bands both bland and bold in presentation. But if you want to listen to the band that truly wallpapered the airwaves with sugar rock, look no further than Van Halen who definitely took some inspiratons from Zep, and paved the way for the formula rock you hear before you today.
But the blues are definitely not dead. Though Grunge definitely threw a shovelful of dirt on it, you can still hear some strains of old school blues.
Unfortunately, it seems that today's pop-country tends to employ it more for their sugar-coated twang rather than applying it properly to truthful music.

Mr(blues clues)Sensitive

THey took the heavey blues sound of Jeff Beck group and went with it.

JImmy page played on Becks Bolero on the first beck group album

Led Zeppelin responsible for bland rock? I think not.
Surely you're thinking of REO Speedwagon.