A Notorious Black Stripe Appears on Models
Often when modeling, you run into a situation where a strange black crease goes along some edges. The stripe is usually most apparent when modeling with the Subdivision Surface modifier turned on. What's happening here is that the normals for one of the faces adjoining this edge are facing the wrong direction.
Usually, the quickest way to fix this problem is to have Blender recalculate the normals for the model and attempt to have them all face outside. To do so, go into Edit mode, select all, and press Ctrl+N. Typically, pressing Ctrl+N alleviates all issues. If it doesn't, however, you may have to go in and manually flip the normals yourself. Manual flipping of normals is easiest to do from Face Select mode with face normals visible. To make face normals visible, enable the Face check box under the Normals label turned on in the Mesh Display panel of the Properties region of the 3D View (N). With that set, a cyan line points out from the face in the direction of the normal. Now you can see which normals are facing the wrong way. From there, select the offending faces and press W
Flip Normals or left-click the NormalsFlip Direction button in the Tool Shelf.
If that still doesn't solve your problem, it could mean that you have multiple vertices in the same place, or you have faces inside your mesh. You can fix multiple vertices by pressing WRemove Doubles. Internal faces are harder to auto-detect, but if you view your mesh in Wireframe viewport shading (Z), it may be more apparent.