From Zelda Legends Wiki
The Blue Ring is first found in The Legend of Zelda (NES) and can be found in the first quest in a shop under an Armos in sector E-4 (5 squares east, 4 squares south of NW most point). In the second quest, it is found in sector P-1. In both quests, it costs 250 rupees.
It later makes appearances in the Oracle series of Legend of Zelda games. In all games, however, it has the same effect: cuts all damage taken in half.