SLR photo black spot

There should be a stain on the sensor, not on the reflector. If there is a stain on the reflector, it will only be displayed in the viewfinder and will not be left in the photo.

Did you send cleaning tools when you bought the camera, including liquid spraying? If not, you can also wipe it with alcohol and gently wipe it on the cloth that cleans the lens. Not once, just a few times. Be sure to use pure alcohol, not hospital disinfectant alcohol, because hospital disinfectant alcohol is not pure alcohol, it contains water.

I found that there are still a few points, the area is relatively large, but not as obvious as this point.