I don't think it will be permanent, in my eyes Blind can play the following positions:
CDM = LB > CB >>> CM
He's a great system player, he will rarely make big mistakes and usually does what van Gaal asks of him without moaning. In a possesion based team he's a great cog. He's not the highest quality player but he's able to slot into 3 (I really don't rate him as CM) positions without much trouble. I don't think that he will get a settled position yet this year, he's far too usefull for that.
The other reason why he played LCB is because van Gaal wants a ball playing CB there I think. He bought Rojo for that reason and Evans used to be pretty good at it who also played there, Blind is the only one on tour who's pretty good at this. I'm sure van Gaal knows the limitation of Blind, I doubt he'll play against Benteke at CB for example. Van Gaal experiments alot but isn't scared to stop if he sees it isn't working. I like Blind at CB for certain matches (where possesion/brain/passing > physicality) but I don't rate him as our starter CB yet. He might prove me wrong but for now I can't see it happening.