You need to first of all work out where the dust or other material is in your imaging train.
Remove the lens and look through the viewfinder of your camera at the sky. This will quickly tell you whether the blemish is on your lens or somewhere within the camera.
Then you need to use a puffer/blower to see if the blem is on the camera's mirror or on the sensor, which you can get to by using the menu to select Manual Clean of sensor.
Also, try blowing some air up into the area directly above the mirror where the focusing screen is housed.
If that doesn't fix it, you might need to look at cleaning all three surfaces (sensor-mirror-focusing screen) to look at removing some kind of residue from their surface.
Hope you get it sorted
