You should use the singular (blueberry) in general when you describe a flavor. I think the hyphens are optional and that any of the following would look ok:
wild blueberry flavored
The fourth option, wild-blueberry flavored, doesn't look right to me, because if you want to connect the words, flavored should be one of the words you connect - flavored is the main adjective that you are describing with other adjectives.
In general hyphens are most useful when they make the meaning of a phrase less ambiguous. There are some good examples of when hyphens are necessary here - e.g. small-state senator vs. small state senator; violent-weather conference vs. violent weather conference. In your case the meaning of the phrase doesn't really change based on how you hyphenate it.