this happened because your normal is flipped.
your scale is -1 in the x direction.
to fix this
1. left click on you dark object
2. Right click on your object
3. Click face
4. Double click on any of the face to select the object
5. switch over to the POLYGON Menu Set
6. In the top menu click on Normal and click reverse and it should fix that
there is a 'fake' fix to it which I don't recommend, because it may cause rendering issues is to turn on double sided lighting.
1. click on the Lighting on the shelf menu and click two sided lighting.
first click on "Modify> Freeze Transformations" in order to zero out the transforms and having the mesh looking grey again. Then click on "Mesh> Combine" to combine them, only now you can merge the vertices, go to "Edit Mesh> Merge Components". Hope it helps, regards, Joel Stutz