I often pick wild blackberries near a river, and I've experienced this for years! Berries from shadier areas result in a more dense but sour berry with a satisfying texture, while berries in the sun taste more akin to sugar and are much, much softer.

I eat the sour ones straight, however I use the softer ones to make jams or crumble filling. Ever had blackberry crumble? Stuff is NECTAR OF THE GODS!