Rovers' winning run might have survived the risk of negating the spell by giving their career caretaker the job until the end of the season, but not playing with 10 men for an hour, as they had to do after the sending off of Jason McAteer.
The Blackburn midfielder, already booked for a clash with Michael Brown, was dismissed for comments aimed at the referee's assistant. He was shown a straight red rather than a second yellow card.
It was the turning point in a match which Blackburn, in their first outing since Parkes was persuaded to take the job on a semi-permanent basis, had looked well capable of winning.
Parkes, Sheffield born and bred and a United supporter in his youth, said that McAteer would be fined for his outburst. "It was stupid and the lad knows it as well," he said. "After that it was a different game and we never had enough possession to win it."
McAteer had played an important part in Rovers taking the lead inside the first 10 minutes, floating one of a series of free-kicks during a staccato opening phase in for Ashley Ward to bring the ball down on his chest - although United suspected a hand - and prod past Simon Tracey.
Blackburn should have scored a second 10 minutes later, again with McAteer involved, as he nodded down a cross from Damien Duff which Nathan Blake, back at one of his former clubs, somehow looped over the bar from three yards out.
It all started to go wrong for Rovers when United fashioned a well-worked equaliser just before the half-hour. Bobby Ford played the ball up from right back, Martin Smith flicked it on cleverly and Bent finished calmly by slotting it past Alan Kelly, another former Blade making a return visit to Bramall Lane.
Two minutes later, McAteer was off and Blackburn's ambitions were reduced to trying to save a point. For some time it seemed that Kelly, out of his sick bed after a bout of flu, might help them to do so.
Although a shot from Brown, on loan from Manchester City, escaped him for Smith to put home, the goal was disallowed for offside and he was equal to anything else, including a deflected shot from Shaun Derry and another effort magnificently saved from Brown.
In the 75th minute, however, he was finally beaten by another clinical piece of finishing from Bent. Paul Devlin's cross from the right was precise and the tall striker got between Blackburn's central defenders for a good headed goal.
Neil Warnock, another new appointment charged with turning a team around, was pleased with the way Bent reshaped the game. "I've told Marcus that he has to work hard and then the goals will come," the Blades' manager said. "He's never worked as hard in his life and he is getting the reward."
Parkes said: "We were disappointed with their second goal, where we allowed Bent to get a free header. But we battled away and couldn't have done much more."
Goals: Ward (10) 0-1; Bent (29) 1-1; Bent (71) 2-1.
Sheffield United (4-4-2): Tracey, Ford (Kozluk, 34), Murphy, Sandford, Quinn, Devlin (Hall, 82), Derry, Woodhouse, Brown, Bent, Smith (Gysbrechts, 90). Substitutes not used: Kachuro, Hunt
Blackburn Rovers (4-4-2): Kelly, Grayson, Taylor, Dailly, Harkness (Davidson, 77), McAteer, Frandsen, Carsley, Duff, Blake (Jansen, 84), Ward. Substitutes not used: Kenna, Ostenstad, Filan (gk).
Referee: M Pike (Barrow).
Bookings: Sheffield Utd: Devlin. Blackburn: Harkness, McAteer. Sending off: Blackburn: McAteer.
Man of the match: Bent.