Although the potion shop in the island with the town makes blue potions using the teardrop items from the slime monsters, it needs 15 blue teardrops to make one blue potion. Instead, use the grappling hook on the monster plants on the island with the big tree to get the venus flytrap item. This item can be traded to the little plant shaman inside the giant tree's cave for blue potions at a better exchange rate (5 flytraps for 1 potion).