Make sure that your blue rectangle is selected and make another two copies in front (Control-C > Control-F > Control-F). Select the top copy and move it 2px down.
Reselect both copies made in this step and click the Minus Front button from the Pathfinder panel. Bring the resulting shape to front (Shift-Control-] ), set the fill color at black, lower its Opacity to 20% and change the Blending Mode to Soft Light.