The issue very probably comes from the UVs. As we can see from your screenshot, some of the UV islands are really small and don't take advantage of all the empty space they have. This will result with a lack of definition, and possibly artifacts.
Make sure to scale them to a proper size and avoid overlapping UVs (which is probably the cause of the black spots).
Best regards,
Cyril Dellenbach (Micro) | QA Support Artist | Adobe