Most likely crud on the mirror which flips up when capturing an image. If after cleaning the mirror the spot is still present then it could be on focusing screen and in that case I would recommend taking the body for cleaning to the appropriate service center.
Bulb blower to start with. If still evident use a recognised technique for your make/model but if u r not confident doing it then get it done by service center.
Crud = foreign object anything not supposed to be there. If you remove then lens and look at the mirror under daylight do you see anything? Dust, pollen, lint, fingerprint etc?
Some dirt on the focusing screen is not so easy to remove just by blowing. My advise to to try to use a blower to blow first and if it doesn't help, send it to the service centre to clean cos manually cleaning it may scratch the focusing screen.