Blue dragon scales respawn by the Blue dragons in Taverley Dungeon. A dusty key is required to enter this part of the dungeon, but if one has 70 Agility, they can cross the pipe right by the dungeon's entrance and arrive directly at the blue dragons' lair. Means to counter dragonbreath attacks are strongly recommended and some food if you are a lower combat level.
Blue scales themselves have no use, but when ground with a pestle and mortar, they become dragon scale dust, an ingredient in Herblore.