Run the blueberry mixture through a fine-mesh sieve set over a bowl or container. Using the back of a wooden spoon, mash the mixture against the sieve to ensure you get all the syrup. Discard the leftover contents in the sieve. Let the syrup cool to room temperature and chill in the refrigerator before using.
For the blueberry egg cream: Pour the milk into a 12-ounce glass and add some seltzer, allowing for about 2 inches of empty space at the top of the glass. Using a long spoon, stir vigorously for a few seconds. Gently pour 3 tablespoons of the blueberry syrup into the glass and then stir again, taking care to stir mostly at the bottom of the glass to incorporate the syrup. The drink should be mostly lavender with a frothy white head on top about an inch high.
The recipe makes approximately 4 cups of syrup. The syrup will last up to 2 weeks covered and refrigerated.
This recipe was provided by professional chefs and has been scaled down from a bulk recipe provided by a restaurant. The Food Network Kitchens chefs have not tested this recipe, in the proportions indicated, and therefore, we cannot make any representation as to the results.