Blind has had some very good games in central midfield, along with some poor ones. As with all new signings, people cut him slack in his first few games so some not so good displays were glossed over. Of course, now he's had some duff games in that position the opposite is happening. People are ignoring the good performances from earlier in the season.
At the end of the day, central midfield is probably the position that takes the most adjusting to. You need to be constantly able to predict where every one of your team-mates will move next and that takes time to learn. So it stands to reason that he'll adapt quicker at LB than he will at CM. He has had some poor games at LB too, by the way. Especially in the FA Cup.