Create a normal patch with a ground texture (or stone whatever..) and copy the patch and place it exactly at the same place. Now give this patch a texture with _blend or _decal behind it.
Now select both patches and press Y. A window will pop up and select flatten and alpha.
The inner and outer radius are the radius and amplitude is if you want to blend it slow or quick, just play with these buttons.
Now in your 3D screen when you hold your mouse on the selected patches you will see 2 circle's, the inner circle (inner radius) and outer circle (outer radius)
Now press alt + left mouse button (on the vertices) and your texture will blend