I don’t remember when this dark dot appeared on photo. I found it few days ago. I cleaned the Nikon lens and reflector mirror to remove it. It was still existed at the same spot even after shooting photos on 300 mm telephoto lens.
There was a dust on the image sensor which I would not expect. Cleaning procedure I did:
1. Remove the lens.
2. Put in Manual mode and set exposure time for 10s.
3. Press the shutter button.
4. Blow air on the rectangular image sensor while checking dust
5. Repeat step 2 to step 4 till image sensor’s surface is spotless.
6. Take sample photos and check the dark dot.