De Gea didn't cost you points against Blackburn directly, if it was 2-0 and he let in a soft goal, it wouldn't have cost you points, he was just one of, pretty much 9 players who cost you the points in that game.
Not sure how you can claim that, the team got the scores back to 2-2 and ultimately the winning goal was due to De Gea's mistake. Had he not made that mistake would we have lost? Probably not. It would have stayed at 2-2 or we may have even gone on to win it. His mistake came so late in the game you cant even blame the players for not going back up the pitch and making it 3-3.
As for VDS I can count on 1 hand the real howlers he made at United