United's offense again wheezed and sputtered as the team's record fell to 2-5-0 and it incurred a second consecutive shutout in MLS play for the first time since May, 2005.
Missing injured midfielders Fred and Marcelo Gallardo, D.C.'s disjointed attacks seldom unlocked Chicago's snug back line. The league's toughest defense (three goals conceded in seven games) logged its fourth shutout.
Twice in the first half United misfired on shots - by Luciano Emilio and Clyde Simms - from just outside the penalty area.
Jaime Moreno, taking the playmaking role in Gallardo's absence, provided a few classy touches in the attacking third, but with Emilio seldom able to hold the ball or break clear of challenges, and Marc Burch and Santino Quaranta laboring fruitlessly on the flanks, Chicago either throttled United's attempts in midfield or repelled its crosses.
The Fire's outside backs, Gonzalo Segares and Brandon Prideaux, contained Burch and Quaranta, which allowed Diego Guiterrez and Soumare to stay tight on Franco Niell and Emilio.
With 18 minutes to play, Fire keeper Jon Busch charged down a shot by Emilio, the second of the two saves he was required to make. In the final minutes, Moreno drilled a header off the crossbar from a corner kick.
Mapp scored in 38th minute when Chad Barrett headed down a Chris Rolfe cross into a yawning gap near the penalty arc completely devoid of United players.
After Niell lost the ball near the center line, Blanco soloed up the middle unfettered before nailing a blast into the top corner from about 28 yards out.
Keeper Zach Wells kept the game close as Chicago peppered his goal in the second half.
Twice he saved from Barrett with the score 1-0 but he couldn't get anything more than a fingertip to Blanco's searing shot that earned the former Mexican international his third goal of the season, the seventh of his MLS career, and no doubt yet another Goal of the Year nominee.
May 8 in Washington, D.C.
D.C. United 0 Chicago 2. Goals: Mapp (Barrett, Rolfe) 38, Blanco 62.
D.C. United - Wells, Namoff, Martinez, Peralta, Burch, Quaranta (Doe, 66), Dyachenko (Kirk, 92+), Simms, Niell, Emilio (Stratford, 83), Moreno.
Chicago - Busch, Prideaux, Gutierrez, Soumare, Segares, Thorrington, Blanco, Pause, Mapp (Woolard, 71), Barrett (Herron, 66), Rolfe (King, 85).
Yellow Cards: D.C. United - Burch 30, Dyachenko 40, Namoff 73; Chicago - Prideaux 77, Segares 92.
Referee: Terry Vaughn.