Ziz said:
I always felt that the blue stun rings were done early in ANH production, before the budget and time crunch started to hit. Once they saw what it cost to do that more than once, they dropped it.

Also remember it's not so much a "stun ring" so much as you are seeing the laser bolt come directly at you and then leave your field of vision. Remember they had to do it in a way that you could still see what was going on (if they used a solid bolt of blue, the whole picture would be blue and thus unwatchable).