Place fresh blackberries, granulated sugar, and 1/4 cup of tap water into a blender. Process this mixture on high speed until completely blended and smooth.

The next step is to place a sieve on top of a large bowl. Pour the berry mixture (I did this in 2 batches) into sieve. Press blackberries down and through the sieve using the back of a spoon. The seeds will stay in sieve, and thick blackberry sauce will drain into the bowl beneath. I found it helpful to use a spatula to "scrape" the sauce from bottom of sieve into the bowl.

Once all the blackberry sauce has gone through the sieve, discard the seeds IN the sieve. For each 8 ounce cup, place 3 heaping Tablespoons of blackberry sauce (or more, to suit your taste). Add ice cubes, then fill the rest of the cup with club soda or sparkling soda. Give the soda a stir, then taste. If you wish the soda to be sweeter, add a bit more sugar (or artificial sweetener), and stir to dissolve. Serve cold; sit back and enjoy a refreshing blackberry soda! (store unused sauce in covered container in refrigerator for 4-5 days).