I had a doctor's appointment after my blackberry-pickin, chigger-scratching episode and told my doc about my misery. She said if it happens again to make an appointment and get a shot for them (may some kind of prednisone or something?); says she has to do it at least once every summer for some...
I had some (like you've got... LOTS) earlier this year, after making the mistake of walking the entire edge of our field picking blackberries. I tried everything to relieve the itch. Nothing works for long. Heat was best; either hot water straight from the tap or blow with a hair dryer on...