For once, United were cheered off the pitch at the end after proving a handful for Gareth Southgate's Middlesbrough side in a high-tempo FA Cup clash.
Now the Bramall Lane faithful are hoping it will spark a late burst in the Championship and push the Sheffield side into a play-off spot.
Blackwell, back at the club three years after leaving former boss Neil Warnock's side to join Yorkshire rivals Leeds United, picked an attacking formation with Jon Stead back in the side to support James Beattie and Billy Sharp up front.
The Sheffield side started impressively and almost snatched the lead when a lazy clearance fell to centre-back Matt Kilgallon, whose fierce strike from 16 yards was kept out by the acrobatic Mark Schwarzer.
United, playing with a new found confidence, went close again with Stead steaming in at the near post to connect with Lee Martin's corner, but his header flashed inches wide of the target.
Middlesbrough showed their Premier League credentials in flashes with Jeremie Aliadiere cutting in from the left flank, but his left-foot strike was snatched at the far post by Paddy Kenny.
They were fortunate to escape after David Wheater pulled back Sharp on the edge of the area.
The big centre-back was lucky to escape with a yellow card, before Beattie hit the foot of the post with a free-kick seconds before the break.
Kilgallon headed a Martin free-kick into the side-netting as the Blades pressed, before Kenny pulled off two spectacular stops in the space of a minute to keep Middlesbrough at bay.
Egyptian striker Mido produced an eye-catching overhead kick which was tipped over the top by the keeper.
Stewart Downing then pumped over a teasing corner which Wheater connected with as Kenny conjured up a carbon copy stop to deflect his header over the bar.
Middlesbrough were sensing a breakthrough and stepped up a gear with the impressive Downing firing in a left-foot volley which curled the wrong side of the far post.
Midfielder Fabio Rochemback then flashed a low strike a foot wide of the target.
Substitute Chris Armstrong drilled a shot over the top and Blackwell brought on Rob Hulse for Beattie in a last-ditch but ultimately fruitless attempt to find the winner.