At the very least he should not duck out of challenge and just stand there not tracking back. He put zero pressure on De Bruyne and then left him for others to deal with. That is a sort of play you expect from a DM who has a cover.
As for hi positioning, it was as bad as it could have been. He should either been further back or closer to De Bruyne with a more chance of winning this 50-50.
I used this video and watched it frame by frame.
When the ball is delivered, Bailly and Blind are inline. Bailly's man goes forward to meet the ball and Bailly goes forward to make the challange. Basic defending. At this point KDB is Pogba's man.
Blind holds his position and turns his body to run into the space behind Bailly anticpating either Bailly to win the header or to dummy it and allow the bounce on to Blind. Every person who has been coached at any level knows that in this instance Blinds job is to cover Bailly's back incase the ball continues. At this point, KDB has gone roughly 4 yards past a ball watching and flat footed Pogba, but is still a good 6 yards away from Blind. He's still Pogba's man, he should be tracked the runner from midfield. If Blind steps up 6 yards to meet KDB and the ball's flight continues (like a defender is told to anticipate) he is running the wrong direction against a player who is obviously faster and already has the run on him.
Next frame. Bailly stops. God knows why. Blind is not god and is unable to anticipate that his combative defensive partner would allow Iheanacho a free header, which is flicked onto KDB's run. At this point KDB and Blind are roughly 3 or 4 yards apart. Watch how Blind is running back towards his goal, he adjusts his footing to make the interception with his left foot, which if anything is his mistake. It's possible that he could have back tracked further but KDB is still just as likley to score by having a free run at a back tracking Blind.
Regardless, KDB wins the ball first and Blind has no choice but to stay still. He makes a challange and he's sent off. We all know Blind can't make that distance back, so i'm sure Blind knows it himself. The only option is it hope De Gea is quick off the line and Shaw is able to get beyond the keeper.
Give Iheanacho credit for meeting the long ball and making a killer flick on. Bailly deserves critism for backing out of the challenge. Pogba makes no effort to track KDB. All these factors leave Blind in no mans land.
His positioning was fine. Maybe it's an instance where his lack of pace was abused by KDB but I think it would be a stretch of the imagination to think any defender would match KDB for pace in that situaiton.
I'm sorry but that goal can't be pinned on Blind. The second goal was all Blind yes.