Originally posted by photolady Well, I've cleaned the mirror front side. I don't know anything about trying to clean behind the mirror.
The dust isn't behind the mirror or on the sensor, because it doesn't show on photos.
Okay, since there haven't been any more helpful on topic replies (yet?), other than the suggestions to ignore the dust, I'll try to explain better.
First, in another browser tab or window, open
File:SLR cross section.svg - Wikipedia, the free encyclopedia
Study the part names on the diagram. Refer to the diagram and part names while reading the rest of this message
See the focus screen, numbered 5 in the diagram? Take a look for the dust there, on the bottom part of the focus screen shown on the diagram. Even if you don't see the dust there, use the blower pointed at the bottom part of the focus screen anyway. Check the viewfinder. Is the dust gone? Great! Otherwise...
The dust can also be on the top side of the focus screen, or on the bottom of part number 6, the condensing lens. To get in there with the blower, the focus screen needs to be removed.
Depending which camera you have (which is?), carefully removing the focus screen can be easy or a task best left to a service center.
Disclaimer: from here onward I am not responsible for any damage(s) you may cause.
If your camera has an easily removed focus screen, there is a little clip retaining the frame that holds the focus screen. Caution here! If you aren't comfortable with doing delicate work, take the camera to a shop and let them.
Before unclipping the holder-frame, read this post
https://www.pentaxforums.com/forums/pentax-dslr-discussion/92167-focus-screen.html#post939697 for how to unclip safely.
Handle the focus screen only by the little tab sticking out the edge of it; don't touch the top or bottom of the screen; it is made of plastic and scratches easily.
Once the focus screen has been carefully removed, check it for dust. Use the blower while holding the screen gently but firmly by the little tab with tweezers. You might also carefully clean it as described in the post about unclipping the frame. I clean mine (Canon 5D1) by dribbling Eclipse fluid on both sides until it evaporates without leaving any streak marks. Distilled water instead of Eclipse will work if the dust isn't greasy.
Look for dust on the underside of part number 6, the condensing lens. Use the blower to remove any.
Re-assemble. Is the dust gone? If not, the dust could be on top of the condensing lens (#6), or between the condensing lens (#6) and pentaprism (#7), or even on or between pentaprism and eyepiece lens (#8), or on eyepiece lens (#8). Getting into those places is a task best left to a Pentax Technician.