Blind side blocks were legal then. (As long as it wasn't in the back but side or front). A QB is protected after an INT except when they "make a play" on the ball. If ref believes he starts to go after returner and is in a position where he may be able to make said play (not 20 yards behind the play), it would be legal. I haven't re-watched it, but ref may have thought he wasn't going after ball.