Add blueberries and lime juice together blender, and pulse until smooth.  (Or alternately, you can muddle the blueberries and lime juice together in a jar until pureed.)  Strain out the juice through a fine-mesh strainer into a cocktail shaker, discarding the berry pulp.

Add the tequila and Cointreau to the cocktail shaker, and stir to combine.

If you would like salt-rimmed glasses, begin by running a lime wedge (the juicy part) around the top rims of your glasses.  Fill a shallow bowl with salt, and dip the rims in the salt until they are covered with your desired amount of salt.  (Or you can skip this step and use regular glasses that are not salt-rimmed.)

Fill the glasses with ice, then pour in the blueberry margarita mixture.  Serve immediately.