Eating blueberries is a different activity from using blueberry bush. Eating berries does not earn an in-game achievement.
When you use blueberry bush then blueberries will appear on it. When blueberry bush is used then its reuse will be possible only after an hour. Back to your question - To get the Bluebarian achievement you must use blueberry bush 500 times.
A little hint. The bush can only be used if it looks like this:
