Cheikhou Kouyate looked to have sealed the points for the Irons before the Netherlands midfielder's late intervention while Luke Shaw saw red in the final seconds
Cheikhou Kouyate, slotting in at centre-back rather than his customary midfield role due to a host of defensive injury problems for Irons manager Sam Allardyce, teed himself up to volley emphatically home four minutes into the second half of what had been an attritional contest.
Despite fielding a side that boasted the esteemed attacking talents of Robin van Persie, Radamel Falcao, Wayne Rooney and Angel Di Maria, Manchester United rarely threatened home goalkeeper Adrian as they failed to dislodge Southampton in third.
Blind cast himself as the visitors' unlikely goal hero two minutes into added time, when West Ham were unable to clear Marcos Rojo's speculative punt forward.
United finished the match with 10 men as full-back Luke Shaw collected a second booking.
Both teams made five changes from their previous outings and there was a nervous early moment for Kouyate in his unfamiliar position.
The Senegal international was sold short by an ill-advised pass from Alex Song inside his own area after six minutes but Van Persie was unable to pounce on an opportunity to open the scoring.
West Ham carried the greater threat during the opening stages and visiting goalkeeper David De Gea was at full-stretch to turn an Enner Valencia drive behind following a poor defensive header by Rojo.
From the resulting 15th-minute corner De Gea frustrated Valencia again by touching an acrobatic effort over.
Robust challenges by Van Persie and Phil Jones prompted touchline fury from Allardyce and, after the latter foul, James Tomkins forced De Gea to save with his legs on the end of Aaron Creswell's floated delivery.
Di Maria thudded a drive into Adrian's chest five minutes before referee Mark Clattenburg called time on a disjointed first-half display by the Red Devils.
Van Gaal's charges appeared susceptible to set-pieces from the home team and that weakness was wonderfully punished by Kouyate in the 49th minute.
Jones and Rooney could not clear Mark Noble's flighted free-kick, leaving Kouyate to flick the ball up twice before swivelling to lash a volley beyond De Gea from seven yards.
A split-second of hesitation from Van Persie allowed Tomkins to make a vital last-ditch challenge in the 57th minute, while a cross by West Ham full-back Carl Jenkinson from the right channel almost snuck in at the far post to double the hosts' lead.
Falcao undermined his reputation as a clinical goal poacher when he prodded wide off balance after a return pass from Van Persie unpicked the West Ham defence.
The roles were reversed in the 82nd minute as Van Persie blasted too close to Adrian from Falcao's pass - Mark Noble proving similarly wasteful at the other end five minutes later.
Noble's miss was one West Ham would regret as Blind rifled a low shot past Adrian before Shaw's late lunge on Stewart Downing provided a marginally earlier end to his evening's work.