Re: Blackberry Picking in Zürich/surrounding area?
Blackberries are just starting to ripen over here on the dark side of the Rossberg, so you might venture out Ausserschwyz way. Mine (in my garden) are still 2-4 weeks away, possibly longer if we don't get much sun in the next weeks.
As well as the Rossberg trails, you might try the Raten Pass to St Jost trail, above the Aegrisee. I remember a good stand of blackberries there in years past.