I think you want to rather feel that your car goes into the sand trap - so, blending textures is one thing, but you would also create some new .rbr files that are attached to driveable surfaces. This means when working with tracks for RBR. These files have surface maps and you can make them with another editor (forgot the name, I away from BTB now, but the program is in BTB root folder - includes also instructions what to do). You would have probably to make some new xpack with these textures, I'm not sure if you can add those new .rbr files (with surface maps) to the project from outside... Also, it looks that the "sandy" surface has to be MAIN surface, then you can mix another material with it, because the "main" surface has priority to choose surface map. I hope you can understand my poor English.
But for rFactor, I have no clear idea since I've never try it.. Maybe in BTB go to "edit materials" and change surface properties to something else (from e.g roada to gravel, grass, etc)