I looked with Wc3Viewer, the textures lie at TerrainArt\Blight\Ashen_blight.blp etc (according to the tileset you use - maybe its the main problem, each tileset has its own blight, and it is needed to point not a single texture, but dependance between tileset and texture). Maybe you could replace some textures in the MPQ but that would mean that your Starcraft mod will include its own MPQ to replace the main (and to return it back when you wanna play a normal map). I doubt there is a way to change something in the map interface. Even trees have their own blight texturing. And trees are linked with Mount Giants who use them (with their native textures!) as weapons, so it stucks deeply.