In order to get the achievement you need to use 500 Blueberry Bushes (even if it's the same bush, for this you will have to wait 1 hr in order to use the bush again), and leave the other 3 berries on the bush, but, you can't use the bush again until someone (You or another person) eats the berries OR you throw the berries on the floor (Thanks to you, now I know that you can take the berries with you or throw them).
So, the Bush has 3 stages:
First Stage (Non used bush):
"You see a blueberry bush."
Second Stage (Used/clicked bush): by clicking the bush it inmediatly turns into (***This is the action that counts to get the achievement****)
"You see 3 blueberries.
They weigh 0.60 oz."
Third Stage (Eaten/Thrown Berries):
"You see a blueberry bush."
This one, doesn't really matter what you do with the berries.
So, the answer to your last question is, NO... You will not get the achievemement by eating 500 berries, actually, you don’t need to eat any berry at all.
Hope's that this helps.