What does main ingredient mean?
If you are making an apple pie, the recipe will ask you to add many different things - these things are the "ingredients". However the largest quantity ingredient will be the apples and this is the "main" ingredient.
Apples are not the main ingredient in blueberry pie; blueberries are the main ingredient in blueberry pie filling; flour is the main ingredient in the crust. It is possible that a commercial product called "blueberry pie" is actually made with apples as the main ingredient with a few blueberries added for coloring and flavor. Although deceptive, a company might do that because apples are much less expensive than blueberries.