Blackwell, who was missing 10 players and could only name six substitutes, saw his side offer a poor showing at Glanford Park as they were beaten by a Grant McCann double and a Paul Hayes strike.
United had briefly been level when Ched Evans, who was one of several players to play with an injury, fired home a free-kick, but Blackwell was not shying away from the disappointing performance.
"It would be too easy for me to hide behind the fact we have had a load of injuries," he said.
"Today we got what we deserved, which was nothing. We were poor from start to finish.
"We have had 10 injuries and I have got a squad of 22. We couldn't fill the bench. But (to use it as an excuse) would disrespect Scunthorpe, who were at it and deserved the win."
Blades defender Matty Kilgallon was taken off after Scunthorpe forged ahead in the second period with two goals inside six minutes, and with transfer speculation constantly surrounding the defender's future, Blackwell insists he needs to focus on his football.
Asked why he took Kilgallon off, Blackwell said: "Did you see the last two goals?
"'Killa' has got to deal with the speculation, it's part of football. You have to play through it and I am sure he is professional enough to play through that, only time will tell."
Scunthorpe boss Nigel Adkins described victory as a "noticeable scalp".
The home side were thoroughly deserving of their three points after a spirited performance but Adkins was keeping his feet on the ground.
"This is a fantastic victory for us," he said.
"They're up there, whoever Scunthorpe beat in the Championship is a fantastic victory - we have to remember where we've come from.
"Great credit to the players, it was a great victory.
"I think we fully deserved to win the game. The work-rate was phenomenal, we tried to pass and we showed character yet again.
"We have been very resolute. I am very proud of the players."