In my opinion there can be four basic reasons for such staining:
- Nutritional imbalance
- Climatic and Physiological
- Mechanical injury and Post Harvest issues.
If it's on all four sides, you may rule out 3 and 4. You may dip a affected pepper in water overnight and spray on few healthy peppers on different plant. If symptoms develop, it could be pathological problem.If no symptoms appear it could be nutritional issue. Now here is my take on this:
Apparently the leaves of your crop are healthy and i do not see any spots, lesions, necrosis or discoloration so assume this could be a nutritional issue. I advise that you may go for a leaf analysis for a more accurate diagnosis.