Why do my imported textures look a lot darker with odd shadows?

When I import a model into Unity the textures look a lot darker and have odd shading/shadows all over them. My scene has a directional light as the sun and the terrain is properly lighted so adding in more lights/increasing the sun's intensity/range isn't feasible (as the terrain then becomes far too bright.)

What's going on here? See attached pictures below. The first shows a wall section as it looks in Unity, the second shows the same wall model but in Blender. How can I make the model look like the one in the Blender shot when in Unity, but still be affected by lights?