It is not unreasonable for a defender to be expected to look at both the runner and where the defender is running. If you hit a defender obliquely so he can't see you even when he looks where he is going that should be blind side.It is silly to determine where the defender is looking, like in your made up scenario. That’s why where the ball carrier is and how the block happens is what you can look at. The defender in these situations are always going to be looking at the runner, which is how blind side blocks occur. If the runner is coming from a different direction, blockers need to be aware of how they are blocking from now on.
In this case, he hit him from nearly the same direction as his direction of movement and in addition the blocker crossed right in front of the runner to get to the defender to make the block so if the defender was looking where he was going and was looking at the ball carrier there is no way the blocker was not in the defender's field of vision.