Yes, I'd agree with blues rock, but is that a subgenre of Classic Rock?
Feel free to talk about it here, as long as it pertains to the topic (which is genre/subgenre). It doesn't have to be specific to ZZ Top.
Personally, I don't think that "classic rock" is a genre or a subgenre but rather more of a "label" or an overgeneralized "category" painted with a very wide brush. There are SO many different styles of music that fall under that category.
Examples:
Southern rock - Skynyrd, Molly Hatchet, Blackfoot, 38 Special, Marshall Tucker, Charlie Daniels (might be stretching it with the last two)
Blues rock - Eric Clapton, Allman Brothers, early Fleetwood Mac, Stevie Ray Vaughan, Jeff Healey, Black Crowes
Hard rock - Zeppelin, Cream, Sabbath, KISS, Scorpions, Nazareth, Rainbow, Deep Purple, Alice Cooper, AC/DC
AOR rock - Boston, Foreigner, Styx, Toto, REO Speedwagon, Journey
Pop rock - Chicago, Huey Lewis and the News, Chicago
I could keep going, but there's no point, this is just for example sake. Now, every one of those bands could EASILY be shuffled into another category (Marshall Tucker and Charlie Daniels into country, Blackfoot into hard rock, every one of those hard rock bands could also be considered 'metal' by some and not even close to metal by other people).
But, the point I'm trying to make is that all of the bands that I listed are all typically thrown under the umbrella of "classic rock". I've listened to classic rock FM radio stations and heard something by the Beatles, followed up by something from Judas Priest, then Bob Seger, then Guns 'n Roses and then U2. every single one of those bands and songs are classic rock, without being even remotely in the same "sub-genre" with each other.
Hope that makes sense.
So, not to dissect my own thread, because I was simply curious what everyone's general or generic categorization of ZZ Top would be.... but to make a point and address your comment.
And think about this, the genres and subgenre's change over time. In the early-to-mid 80's, Motley Crue, AC/DC, Scorpions, Van Halen, etc were considered "heavy metal" or were at least grouped as being heavy metal. Over time, there's very little about any of those bands that would be "metal" by today's standards.
Edit; By the way, I'm not saying this is 100% fact and the way that it is, just using examples for the sake of needing and using examples.