Place the berries, water and agave in a small saucepan. Cover loosely [tilt pan lid to allow steam to escape or it might boil over] and simmer over low heat for about 10 minutes, until the blueberries are completely cooked and broken down.
Immediately, place a mesh strainer over a small bowl, and strain the syrup, running a spoon back and forth through the berries to make them run through, and pressing on the solids at the end, to extract all of the juice. Refrigerate, or if you are in a hurry, stash it in the freezer for 20 minutes.
Make the blueberry salt
Place the salt on a small plate, and completely stir in the the 1 teaspoon of the blueberry syrup. A fork is the easiest way to do this - kind of mash them together.
Spread the salt out on the plate to let it dry a little bit, though this is optional.
Make the Margarita
Slide a small wedge of lime around the rim of each serving glass, and immediately rim the glass in the blueberry salt, by sliding it around on the salt on the plate. Set aside.
Measure the tequila, lime juice, blueberry syrup and triple sec into the bottom section of a Boston Shaker. Add 3 or 4 ice cubes, place the metal glass on top, give it a firm tap, turn it so that the glass side is up, and shake vigorously, until the metal is nicely frosted. Keeping the glass side up, tap the side of the metal glass on the edge of the counter to release it.
Pour the drink into the rimmed glass, plop in a couple more ice cubes, some berries and slice or two of lime.
Notes
Best served immediately, though you can increase the ingredients proportionally to make a pitcher of Blueberry Margaritas for a party. The amount of syrup in the recipe will make about 8 margaritas.