If the black areas are not totally black, back off a little more. Now slowly turn the contrast control down. At some point, the darker and lighter colors will seem to snap into a normal relationship, with the white areas remaining bright white and the black areas still solidly dark. At this point you should check the setting of the brightness control again, but do not move it very far in either direction. The second kind of adjustment enlarges the display to fill most or the entire screen.
Most monitors come from the factory with an unlit border around the image area, for no good reason.
Every monitor but the cheapest ones provide buttons or knobs that enlarge the picture horizontally and vertically. You'll undoubtedly also find controls that shift the picture up or down and to each side. With your normal desktop showing, expand the horizontal image area until it almost fills the screen. Do the same for the vertical adjustment. You'll probably have to shift the overall image sideways or from top to bottom while you are doing this to keep it centered.