Some additional things to try....
Turn the camera off. Is it dark when pointed at a reasonably lit subject?
Take the lens off, is the viewfinder still dark when pointed a reasonably lit subject? It won't be in focus of course, but you should see the brightness of the scene.
You could have something wrong in the camera if its dark without a lens, like with the mirror out of place. If its bright without the lens, then you need to see if something is stopping down the lens. If its with multiple lenses, it could be a camera problem.
Take the lens off the camera. On the mount end, there is a small lever. You can gently push this lever to open and close the aperture. I think for most lenses, spring tension stops down the lens (aperture closed). Observe how this looks as you open and close the aperture. Look with a light into the front of the lens and observe. Mount the lens to the camera and with a light be sure the aperture is fully opened.