When I make blackberry jam I always put it through a mouli to remove the seeds. I don't know if that improves the flavour, but making a lot of jam for our allotment shop I've found our customers prefer it seedless.
charley26, I have always brought the fruit to the boil to dissolve the sugar and then left it overnight as I found it really improved the set. This was confirmed recently on The Kitchen Cabinet, Radio4 (if my memory serves me well), although I can't remember the science; something to do with the pectin I guess.