Authoring grass and landscape textures to blend nicely can be pretty tough at times. This is where automated methods come in handy.
For the following material to work you'll need some kind of a terrain base color texture which is then being sampled in the grass material. The most important part is to make it match the actual landscape size, scale and location.
Once this is done you might find various ways to alter the grass color based on the underlying terrain. In this example I'm using a simple vertical gradient to blend between both textures.
Regular Grass Material
Terrain Blending Grass Material