Blackberries

It's obvious why they're called blackberries, but why aren't raspberries called redberries?

Blackberry-Picking
BY SEAMUS HEANEY
for Philip Hobsbaum

Late August, given heavy rain and sun
For a full week, the blackberries would ripen.

Sounds a bit like a bank holiday weekend to me!

You can find the full poem here: http://www.poetryfoundation.org/poem/181384