blue berries - how to tell when they are ripe?

[gardeninglee]

I feel like these blue berries cherry tomatoes have been on the plant forever! I've picked probably a dozen or so sunsugar already. I've picked two of the blue berries thinking they were ripe but they were super bitter and green inside. I'm about ready to pull them out but want to taste one ripe one to be sure it's not worth keeping. How can I tell when they are ripe?