Instructions
Go to town and visit Costolots General Store. Purchase at least one blueberry seed, though if you get more, you improve your odds of growing a bush successfully.
Return to your garden, select the blueberry seed and then plant it.
Water the seed daily. After a few days, it will sprout and an emoticon will appear over it. Your goal is to keep the emoticon smiling. If it frowns, water it.
Harvest the blueberries once the bush has fruited.
Go back to town and visit Bart's Exchange. Give him the blueberries and he'll give you muffins.