why are blackberries so high in fiber?
It could be that it's the quantity that is misleading, since the quantity of one cup is quite a large amount of blackberries. Some of the other quantities listed are more reasonable, for instance, it lists "five dates" or "ten grapes". A whole cup of blackberries at one setting would give a normal person what my father called the "Quick Step". You learn to step quick after eating that many or you will never make it.
I found a dietery link with a table showing a separation of the soluble fiber amount from the insoluble amount. It will add another piece to the puzzle.
Scroll to the middle of the page for a table comparing fiber content in even more foods
I can eat far more than that with no digestive problems. I'll often graze on wild blackberries (brambles here) when they're in season, and can make a full meal out of them with absolutely no ill effects beyond feeling full for a bit and needing a piss. Other berries don't give me problems either.
I said a normal person, Peter.
You're also talking about native brambles, which you are used to, apparently. Most people who I know to be heavy meat eaters, like my dad was, could never eat many fruits. I would not be surprised to find a connection, there.
Brambles don't grow everywhere, either. They are truly foreign food to some people. I don't have a problem with berries, either, but apples I do. If I eat more than a whole apple, raw, I am in danger of contracting the Quick Step. Other fruits in any quantity ... no problem. It's just something about raw apples.