Are the black borders part of the desktop i.e. can you put the mouse cursor or icons into the black area, or is the black area 'wasted space'.
If the black area is 'wasted space' around your desktop then it may be that you have to adjust your monitor settings, picture size etc, to fill the screen again.
If you can move things into the black areas i.e. the desktop is full screen but the picture isn't, then it may be the type of wall paper in personalization, its 'picture position'.
right-click on your desktop.
click personalize
towards the bottom, click 'desktop background'
towards the bottom is a pull down selection list called 'picture position'.
It has options such as - Fill, Fit, Stretch, Tile, and Center
Try each in turn and see if it helps.
If it does then <save changes>
If not, have you loaded any new updates like new graphics drivers lately?