If I rinse off a fruit (like blueberries) and then mix it with yogurt, do I need to do netilas yadayim?

No need (assuming you are using a spoon), because any food (fruit in yogurt) that is usually eaten with a spoon (or a fork or other implement) does not require Netilas Yadayim.

Sources:

אף שהמים הם מז׳ משקין המצריכים נטילה, דלא כהיוגורט, מ”מ דרכו להאכל ע”י כף – סדר נט”י סעיפים כ-כב, שו”ע אדה”ז קנח, ד.