"michael adams" wrote

There was also a superstition that backberry juice
represented the blood of Christ and that the fruit
shouldn't be eaten at any time, on account of that
belief.
michael adams

Bit more trivia from http://www.mwrop.org/W_Needham/Blackberry_050807.htm :
The medicinal benefit of the blackberry extended to superstition. In
medieval England, the arch made by a bramble that rooted at both ends was
believed to provide a cure for hernia or rupture in children; the afflicted
child was passed backwards and forwards through the arch. A similar
treatment is still in use in Cornwall, where children are passed through a
slit in the trunk of an ash sapling which is afterward bound up to affect a
cure. A bramble-cure of this sort was also used to heal boils, blackheads
and rheumatism in adults, who either crawled or were dragged beneath a
bramble shoot. The name scaldhead for the blackberry is due to the belief
that children who ate the fruit to excess became afflicted with a disease of
the scalp called scald head.

Jenny