The blue apple is a rare drop from apple trees.
The game may produce one or two blue apples per day.
For a blue apple you'll get 4 coins and it takes 3 seconds to sell.
The blue apple can be used to craft a drink at the Seadog Cove Tavern on Seadog Cove.
The grogmaker will turn the blue apple into grog which can be sold at the Food Stall.
The blue apple can be used to craft Blue Apple Juice at the restaurant in Larona Village.
The recipe requires three blue apples to create and when finished it can be sold at the Food Stall.
The restaurant owner sometimes requests three blue apples in one of the side-quests, so it may be beneficial to store some in the Warehouse.
The blue apple can be used as an ingredient for Purple Paint which is needed to reconstruct some buildings.
The Purple Paint is crafted at the Bullhorn Farm Windmill and requires one blue apple and two blackberries.
When completed the Purple Paint can be used to create a building, or sold at the Woodwork Stall.