"I thought they all acquitted themselves very well," said Blackwell.
"I give credit to the lads, especially the likes of Lee Williamson, Henri Camara and Jamie Ward who've all only recently returned to action. We knew they were going to tire."
Blackwell was also delighted with the performance of penalty-saving goalkeeper Mark Bunn.
"Bunny's had it tough lately because he's found how difficult it is to play for a big club, but I thought he put in an accomplished performance. I'm delighted for him because he's a smashing lad."
New Peterborough boss Mark Cooper, who took charge of the side for the first time since replacing Darren Ferguson, felt they were unlucky to leave Bramall Lane empty-handed.
He said: "We definitely deserved a point. It was a pretty even first half and I thought we were the better team in the second half.
"We looked quite solid as a team - especially at the back and in the middle - so there was plenty to be pleased with."
Cooper refused to blame Boyd for his failure from the spot, commenting: "We're not going to criticise George because I'm sure he'll win us plenty of games."
Despite remaining rock-bottom of the Championship, Cooper feels he can steer Posh to safety.
"I'm more confident after seeing that than I was during the week," he said.
Peterborough threatened early on with a low drive from Boyd flashing narrowly wide of Bunn's left-hand post and Chris Whelpdale putting a 20-yard effort just over.
Blades striker Ched Evans went close to opening the scoring when his shot crashed against the underside of the bar before bouncing away.
The only goal of the game came on 42 minutes when Camara followed up to turn home the rebound after Evans saw his effort parried by Lewis.
Peterborough threatened Bunn's goal on several occasions after the break.
Craig Mackail-Smith created space for himself inside the area, but he only managed a tame shot which was easily kept out.
Mackail-Smith was again in the thick of the action, heading wide from a Scott Griffiths cross.
Peterborough's best chance to score came in the 67th minute when they were awarded a penalty after Mackail-Smith was fouled by Andrew Davies inside the area.
Boyd's spot-kick had plenty of power, but it was kept out by Bunn who made a good save low to his left.