Add blueberries, lemon zest, lemon juice, and sugar to a medium saucepan. Mix well. Place over medium heat. Heat until it comes to a gentle boil, stirring occasionally
Continue to cook over medium-low for a few more minutes until the berries are very soft and have released their juices.
You have 2 choices at this point, either strain the skins out or simply blend them in a food processor. I blended them. If you want a smoother float, strain them out.
With either method you used, add the blueberry sauce back into your saucepan. Now we will start the reduction. Cook over a medium-low, stirring occasionally until the sauce has reduced slightly and thickened slightly. Mine took about 10 minutes. This will depend on the sauce, the thickness of your pan, and the actual temp of your medium-low, as everyone's cooktop is different.
Once it has thickened and reduced, remove it from heat. Pour into a bowl or container to cool. That pan will hold on to residual heat, so I like to remove it from the pan.
Once it has cooled, you can use it right away. If you want to save it for another time, this is simple. Store in an airtight container in the fridge. I used a mason jar. It will hold for up to 4 days.
When you are ready to use, we will make a mix of blueberry reduction and soda. The exact ratio will depend on your palate and preference. I used 1 part blueberry to 2 parts soda. Taste as you make and adjust as needed.
Next, add your ice cream to the serving glasses. We like high ice cream-to-liquid ratio, so don't skimp! Then pour your blueberry soda mixture over the ice cream and go for it!