This is probably a weird and very simple question, but how do I get the dark blotches on the ground around buildings that look like shadows? Also, how do I delete them (I try to retexture the ground, but the dark spots remain, it just darkens the new texture)?

In the Landscape Edit window click Edit Colors.
Now the Left Mouse button will paint the left Vertex color, and the Right Mouse button will paint the right Vertex color.
By default, Left will be a shadowy-black (perfect for what you want) and Right will be totally clear. You erase any coloring with the clear color.

Awesome thanks!!!