If your mesh has dark shadows, it means that you have not specified smoothing groups correctly in your 3D program. Wherever black shade appears, this indicates two or more hard edges of your mesh are being treated as a soft, curved edge.
Where the edge is in fact a hard edge (for example: the edges of a cube as shown below), the faces of the cube on the right have all been welded into a single smoothing group, and the program is now trying to render the entire cube as a round shape! The cube on the left is correctly welded with adjoining faces placed in different smoothing groups.
When black shadows occur, the fault WILL be shown in the game! The answer, however, is not to unweld all vertices. Although a quick solution, it will give any curved surfaces hard edges too, and it will increase your vertex count dramatically (see also: What is the maximum polygon/vertex count for objects?), meaning your creation will use more memory than if it was optimized correctly (smoothed and welded). Instead, you need to divide your meshes into groups of faces that can be smoothed together (see: What are smoothing groups?).
Generally speaking, when you use any premade primitives (cubes, spheres, cylinders etc.), these shapes have been computed to assign their faces into correct smoothing groups automatically, so you will ordinarily only encounter this dark shadows issue with your mesh when joining two smoothed shapes together, or when creating your own shapes from nothing.