Combine the sugar and water in a pot and bring to a boil. Cook until the sugar is dissolves. Add the lilac blossoms and the frozen blueberries if using. Let the syrup simmer for a couple of minutes then turn off the heat, cover, and let steep for 15 to 20 minutes. Strain the syrup through a fine mesh sieve and let cool completely before using. This makes about 3 cups of simple syrup.