Normal map (Bump Diffuse) turns objects dark.

I am trying to wrap my brain around this however, I am not sure where to start.

I created my meshes in Blender and import them into Unity. They look great when I place the color texture on them, however, when I place a normal map (using Unity) on the mesh it changed the lighting on the entire mesh.

For example: I have 5 seperate sections of sidewalk with a cement texture on them, when I place a bump diffuse (then apply the normal map texture) on the material (in Unity), some of the 5 sections of sidewalk turn really dark, however, some of the sections look great.

I have checked the normals. THey are correct (the color textures look great)

Like I said, it is only when I apply the bump diffuse (normal map) and doesn't seem to happen to all of my meshes. Any suggestions?

@Ned Had the same problem, Gimp + normal map plugin => dark textures. It’s hilarious, make sure you mark the texture as normal in Unity Inspector - Import Settings, otherwise if you use it for terrain, Unity will NOT complain that you have not marked it properly (as it warns for models). Thank you for helping me solve this, I had some great pavement textures with no normals, such a pity.