I have noticed walking to work some bushes growing among some roses.
They have tiny black berries (anoput the size of a raspberry). The
stems are covered in small, almost hair-like prickles but no true
thorns (like proper blackberry bushes). The berries are much smaller
than regular blackberries and taste very tart (yes I know I took a
chance there but they seemed fine) but otherwise look just like
blackberries.

Any idea what these are?