Strange dark patches on model

Novice Blender user, puzzling over a model.
Edited part of an item from an online library. At some point in the process the dark patches you can see above on the knees of the legs appeared - have no idea when, and no idea at all what is the cause.
Normals are correct - no internal geometry - all faces are set to smooth shading.
The faces themselves appear exactly as they did in the original model, so far as their actual geometry is concerned - there seems to have been no change. Except now when rendered, it has these diseased-looking areas.

Anyone able to help explain or offer advice?

Clear the custom split normals Data

Immediately solved my problem, and now I have learned what Custom Split Normals are!

Many thanks, much obliged.