The Texas blackland prairies ecoregion covers a main belt of 43,000 km2 (17,000 sq mi) and two islands of tallgrass prairie grasslands southeast of the main blackland prairie belt; both the main belt and the islands extend northeast/southwest.
The main belt consists of oaklands and savannas and runs from just south of the Red River on the Texas-Oklahoma border through the Dallas–Fort Worth metropolitan area and to Austin and San Antonio.
The larger of the two islands is the Fayette Prairie, encompassing 17,000 km2 (6,600 sq mi), and the smaller is the San Antonio Prairie of 7,000 km2 (2,700 sq mi).
A good place to see what might be near you is here at the Native Prairie Association of Texas website for the Blackland Prairie, which broadly lists all North Texas prairies, including some blackland. The largest remaining tract of blackland prairie is Clymer Meadow, owned by the Texas Nature Conservancy east of Dallas near Greenville.