It's fairly common for graduated girls faces to be blurred. Not sure the reasoning behind it but the definite reasons are:

• Girl went through a scandal and left on bad terms
• Girl went into AV

Other reasons as to why members faces are blurred out were:

• They were no longer under AKS or BINGO does not have permission to display their likeness unless the actress gets paid, so there has to be some negotiation to display their faces.

On many occasions when they displayed the first intro where Noro Kayo and all members say "AKB 1JI 59FUN!" and those faces are not blurred, but other times they are in other scenes. The BINGO staff is very inconsistent with the blurring of faces.

So we really don't know as to why some are blurred. But definitely if they went into AV they won't have their faces shown.