Make sure that your blue rectangle is selected and make two copies in front (Control-C > Control-F > Control-F). Select the top copy and move it 1px down using the down arrow button from your keyboard. Reselect both copies made in this step, open the Pathfinder panel (Window > Pathfinder) and click the Minus Front button. Bring the resulting shape to front (Shift-Control-] ), make sure that it stays selected and focus on the Appearance panel. Set the fill color at black (R=0 G=0 B=0) then simply click on that "Opacity" piece of text to open the Transparency fly-out panel. Lower the Opacity to 20% and change the Blending Mode to Soft Light.