Blue dragon scales spawn in the Taverley Dungeon and the Taverley dungeon resource dungeon, and they are currently worth coins each. The better place to collect them is the resource dungeon as they're closer together and there are more of them, but that area requires 60 Dungeoneering to access. Without the resource dungeon, you can collect about 550 scales per hour, compared to 800 inside. 70 Agility is also recommended to avoid having to traverse Taverley Dungeon, which would require a dusty key. A Beast of Burden like the spirit terrorbird, war tortoise or pack yak are also very helpful to cut down on banking time.