In the 28th second they show the replay of the pass to Costa. Everyone other than CBJ are in one line. Costa is clearly behind blind played onside by CBJ. What happened next is actually completely irrelevant because even if he hadn't slipped going to cover terry, he wouldn't have bothered to move like anyone else as they had (or they thought) just played him off and expected the play to be stopped anyway. There's nothing stupid about that. It's the most sensible thing to do and what we would see most intelligent defenders like Maldini baresi and all do.
Lets break it down:
23 seconds: Blind and CBJ hold out their arms as if to say "this is our line" Blind is actually marginally behind CBJ, but that's neither here nor there. Important thing to note is that Darmian, instead of looking around and seeing who he could mark, just decides he'll walk back over to right back... not a great idea considering where the ball is, and the fact that we had men over on his side.
25/26 seconds: With Darmian wondering right, Blind has two men to pick up. Him and CBJ are still in a line together, but Blind decides he should mark Terry, not Costa (the incorrect decision for a number of reasons, not least because you should always pick up the player closest to your goal) - so he begins his sprint.
26/27 Seconds: Blind has sprinted away from CBJ and even past Martial. Leaving Costa unmarked and alone. There is absolutely no chance in hell that CBJ would a) know that Blind was going to choose that exact moment to sprint out of defence - especially as moments earlier they both put there arms out to indicate where the line they were holding should be - and b) could react in literally a second to match his movements and step-out... if you want to be critical, you can say that CBJ isn't even watching the play, which is poor from him, but even if he was, do you think he could have matched Blinds run and moved to be in line with Martial?
28 Seconds - The ball goes through to Costa, Blind realises he's moved the wrong way and falls over trying to adjust his body position. He was nowhere near the ball though (for people saying he'd got it if he hadn't slipped, freeze the clip at 35 seconsds - he's a few yards away from the ball). Costa - the man he should be picking up - has a free run at goal.
People can say all they want "Well what if Terry gets the ball? shoots and scores?" but a) that didn't happen and b) I would rather have John Terry have the ball outside the box then Costa have a 1vs1 inside the box. Ultimately though, if Terry gets the ball, then that's a new situation and we defend the new situation accordingly.
As people have said, the fact that Schneiderlin takes up a naff position isn't great, and yes, you could argue that CBJ should be more aware of whats going on,
But he has such little time to react to Blinds movements (if you want to maintain he was playing offside AND stepping out to Terry fine, though you at least have to concede it's a massive risk on his part and not one he should be taking) ... ultimately, considering the fact that Blind wanted to hold a line where he and CBJ were literally seconds before he sprinted away, makes me believe he wasnt' playing for offside and instead tried to guess what was going to happen and went to go stop it (the move of a midfielder).