The easiest way to clean the upper parts of a Black & Decker lawn mower is with a damp cloth. To clean the underside of the deck and the blade, you should use a lift to raise the mower safely. Unscrew the nut keeping the blade in place, and take the blade off. Remove any stuck grass blades and clean dirt and other debris with a brush. You should not use a garden hose or pressure washer to clean the mower, as you might damage it.