lol Bro, ZZ Top is a straight up blues-rock musician influenced by the music of mostly black blues artist, especially Texas blues artist like Lightnin Hopkins and Albert Collins who both played out of Houston, not a country musician whatsoever. Dude even takes influence from local Houston rap. Iono who told you he was a country musician.
In and Out ain't actually in Houston. It's in the the suburbs of Katy and Stafford, where yes, a lot of transplants live. Just like Atlanta suburbs have NY based restaurant chains like Ruby Tuesdays.
Having a strong state identity and pride doesn't take away from the southerness of the place. People from Louisiana, especially southern Louisiana, have a very strong state identity, and'll holla Louisiana before being southern. And people from New Orleans?!?!? They might as well be in a completely diff country according to them. They're New Orleanians first and foremost before anything.
Even in the whitest areas of New Orleans, they don't be flying no confederate flags. They fly that Gold and Black crescent symbol looking thing and rep those gold, purple, and green mardi gras colors.
Yet and still none of them are under any illusion about not being louisianians or southern nor is anyone else. Same thing in Texas. Our state has a unique history that we're proud of(that black texans were very much apart of), but they doesn't make us any less southern.
And while Texas ain't as southern as GA overall. Don't underestimate it's southernness as it's still home to more historic all black freedmens towns and settlements than any other state. Houston proper is home to at least 11 of these which were built from the ground up by freed slaves and their children, and are lined with homes built in the traditional AA shotgun style. I know Atlanta has some, but I'm pretty sure they don't have as many.
And like I said the vast majority of black transplants in Houston are from other places in the south mostly Louisiana and other places in Texas just like it's always been. California may be the origin of most of the NORTHERN black transplants, but not overall.