Texas Blues are either songs and music that are made in Texas or Blues made by Texas artists who then take it on the road or overseas to other countries. The audiences around the world can make a distinction about Texas Blues, because the songs speak of or tell the story of "some happening" in the Lone Star State, say ZZ Top's "La Grange", T-Bone Walker's "Trinity River Blues", or Townes Van Zandt's "Rex's Blues".
The two principle areas producing Texas Blues are the Dallas / Ft. Worth area and it's surroundings and the Houston area and surrounding cities, with Austin and San Antonio also producing native blues in smaller numbers.
Rural cities and small towns also produce Texas Blues, like Bastrop, Texas which produced Roosevelt Thomas "Grey Ghost" Williams in December 1903, who in 1940 recorded "Hitler Blues" which was heard all the way across the Atlantic on the BBC (England) during WWII.
Some out of state artists, like Robert Johnson, born May 1911 in Hazlehurst, Mississippi, recorded only in Texas! In San Antonio (1936) and in Dallas (1937), Johnson recorded 29 different blues songs, his entire catalog. Thousands of blues guitarists have played his songs over the decades that followed his death in 1938 at the young age of 27! Are they all playing Texas Blues? Very possible. "Walking Blues", "Riverside Blues" and all the rest, they were all originally "made in Texas". They might be considered Texas Blues also, as some of Johnson's songs have "the feel of driving under the stars, late at night on a dark, lonely Texas dirt road."