The major populated areas of Texas are all Southern: DFW, Houston, SA-Austin...
Dallas is said to have a slight Midwestern air, but I didn't see that when I was there. Maybe in the white communities...
SA-Austin area has more of a slight SW vibe, but I've known a ton of blacks from here, including some fam of mine who live there. Blacks in this area are southern, no question about it...
Are they 100% southern? No, they are heavily influenced by Mexican and SW culture too, but to say Texas ain't the South....somewhere around like 75% of Texans live in the DFW, Houston, or SA-Austin areas. That area is more aligned to the South than anything. The area where the 25% lives is less southern but is less influential on Texas culture anyway...