Blake has goals in four of five games for the Islanders, 6-1-1 in their last eight. Washington, still smarting from a 7-1 home loss to Toronto on Friday, lost for the sixth straight time - including two overtime defeats.
Richard Park and Trent Hunter also scored, and Alexei Yashin had two assists for the Islanders.
Alex Ovechkin had the lone tally for Washington, scoring early in the third period.
DiPietro denied two earlier scoring chances by Ovechkin, last season's NHL rookie of the year. The flashy forward twice fired pucks from the slot that DiPietro stopped. Eric Fehr also was turned aside when DiPietro quickly slid across the crease and handled his one-timed shot from in front.