In the following tutorial you'll learn how to add variety to your terrain by applying a texture that has blend properties and painting in the alpha.
1. Painting Alpha Terrain allows you to blend between two textures thus adding more variety to your displacement.
2. Apply a blend texture to your terrain brush. You can find blend textures by filtering textures with blend or nature in the key filter.
Anything that you see below as an example has alpha/blend properties.
Apply to your terrain displacement.
3. Select Face Edit Sheet and click on Displacement tab, Select your terrain and click on Paint Alpha.
RMB/LMB: paints in/out the blend texture.