Just another point I should have mentioned.
There is nothing wrong with taking a value out from the end of the function to drive displacements. The reason I didn't do that is because I simply thought the results were a little severe and less natural. It will go from 0-1 within the blend region. Displacements will only happen there or at least provide variation there. It will be like a kind of step. However, this may be exactly what you want. I might try using some function there to soften the displacement effect. The displacements will also line up tightly if you do this but they won't carry on up the hill or down, they will create a step.
Also with that method, voronoi will get a bit weird because it will kind of invert through the blend. Perlin doesn't go so weird in this scenario.