Cut the Key limes in 1/2. Set 4 aside. Put a blueberry or two with the Lime halves on 4 skewers for garnish
Squeeze the rest of the limes into a large pitcher. Leave the cut limes in the pitcher. Add in the Blueberries, coriander seeds, and cilantro. Muddle these until well blended. Add the agave and stir well.
Add in ice, then the rest of the ingredients. Stir well. Then, pour the Margarita into glasses rimmed with sugar and salt. Garnish with the skewered lime and berries.
Notes
This is a sort of hybrid margarita/Mojito. You can pick out the key limes for a cleaner presentation. But, this gives a more rustic look and feel to the drink.