Yeah, blackberries can be an agressive weed, I know ! I sometimes wonder why the whole planet isn’t covered in them yet, given that they’re such agressive growers. It must be that they need a very fertile soil.
Mangetout, I had meant to point out the difference between dewberries and blackberries. Like you, I thought that maybe elbows is dealing with a dewberry (Rubus casius), or a dewberry hybrid, instead of a blackberry (Rubus fructus spec). The berries of a dewberry have a “frosted” appearance. Compared to blackberries they taste much more sour, even if they are ripe. Blackberries, dewberries and reaspberries are not totally different species, and can form hybrids. The best thing to do with such an unsatisfactory hybrid in your garden is probably to remove it and plant a storebought blackberryvariety with sweet berries and less thorns. Within three years, it will be just as gigantic a bush.
I assume everybody has heard the warning that no blackberries should be eaten uncooked, especially not blackberries that grow in areas populated by foxes, and especially not berries that grow close to the ground (say less then a metre?). There’s a slim chance they are infected with fox-tapeworm. With berries growing above your waist, or berries that are cooked, there is no danger.
Oh, and Ponder Stibbons, yup, we were stained all over. And we tasted good, too !