I finally picked my first bit of blueberries (Reka, I think? either Reka or a plant I lost the label for) today. I left them on the bush for several days after they turned blueberry blue, but I’m not sure they are really as sweet as they could have gotten.
How do you tell when they are at the peak of ripeness?
It’s all by experience. Mine usually stay on the bush until nearly all turn blue. So that 2-3 weeks after the first turn blue. The quality seems to hold up on the early berries. I’ve left some on Southmoon as long as 6 weeks but that’s unusual. Sweetcrisp stays crunchy even when the berry is shriveled.
Shake them or pull lightly. If they fall off they are probably fully ripe. If they are in large clusters I roll them slightly and take the ones that fall off. They don’t all ripen at the same time. The ripening can last for several weeks.
The birds know best, LOL, each variety is different, it helps to know how dark each variety is supposed to look, I have not figured it out yet because ours are still low production and even when they are raw something is eating them on us, most likely squirrels, they are not picky at all, they steal raw tomatoes here.
Birds do not know best. Birds are the greediest. They start eating fruit, not just blueberries when the fruit just turn color.
Like others said, if they come off easily, they are ripe. If you have to pull them, not ripe, even though they may look blue to you. Some varieties ripen over time like Chandler. Don’t pick them all at once.
I pf I were you, I would pick the bluest one and try. If it is tart, wait a few more days. You need to cover your blueberries. Both birds and squirrels will get to them before they are fully ripe.
My experience as well. They started on my Fuji apples in early August, ripe in Oct. Grapes a month early. Squirrels eat all the pecans before the shucks split when the meat is still soft, etc, etc…!!
Don’t forget cherries. Some gone when they are still green 
Some blueberries you can tell as the area near the stem stays greener. When that is blue, they are ready. Not all are like this though. I have Legacy which has berries ripening for 6 weeks, and the stem area turns blue right away. The one thing I don’t like about them. Otherwise they are great, firm, good, vigorous plant that can even take late freezes and still produce berries. Last year blossoms got hit with a freeze and temps down to 21F, and the flowers were formed, and still all produced fruit.