While rendering a mesh with smooth shading on, I get a dark region/shadow on certain faces. The effect is more evident when smooth shading is applied to a mesh containing ngons, with the ngons turning black when rendered.
The blackening effect is visible in the viewport too.
How can I solve this problem?
Check your normals. If some of the faces are turned inside out, pointing inward into the inside of the mesh, they can be seen as dark areas.
You can recalculate the normals in Edit Mode using ShiftN or under Mesh > Normals > Recalculate ... (The entire menu can also be accessed by pressing AltN).
After recalculate normals.
Note that normals will only be recalculated for selected faces, so you probably want to select all (A) before recalculating.
In Blender, each face has 2 sides: one is considered the outside and one is considered the inside. The Face Normal indicates the outside face.
Blender displays faces differently depending on whether or not the normal is facing the viewer.
If you want to view your normals, you can open the Properties panel (N) and in the Mesh Display section, click on the Face Normals button:
(NOTE: Vertices also have Normals. You can view those by clicking on the Vertex Normal button here)
You can then flip normals of selected faces while in Edit mode by:
3D Viewport Header -> Mesh -> Faces -> Flip Normals
Or, if you want to quickly fix all the normals in your mesh, while in Edit mode you can use Recalculate Normals:
3D Viewport Header -> Mesh -> Normals -> Recalculate Outside (CTRLN)
3D Viewport Header -> Mesh -> Normals -> Recalculate Inside (SHIFTCTRLN)
