Heat up a gas grill, or a grill pan on your stovetop. Make sure the grates of the grill or grill pan are very clean.
Pour the granulated sugar in a shallow dish. Lay the pineapple planks in the sugar and coat them in sugar on all sides.
Brush the grill grates or grill pan with a light coating of oil. Once the grates are hot, lay the planks of pineapple across them.
Grill the pineapple for about 4 minutes on each side, until the pineapple starts to look soft and has grill marks. Remove them from the heat.
Freeze the pineapple, and make the garnish:
Once the pineapple has cooled some, place the pieces in a Ziploc bag or sealed container. Freeze the pineapple for 4 hours.
Meanwhile, place two margarita glasses in the fridge. Thread the blueberries onto two cocktail picks. Wrap and refrigerate the picks until you're ready to make the drinks.
Make the drinks:
Pour some lime juice into a shallow dish, and the salt or sugar into another shallow dish. Dip the rips of the glasses into it the juice, then roll the outside of the rim in the salt or sugar to create the coated edge.
Place the pineapple into a blender along with the crushed ice, tequila, triple sec, and 2 tablespoons of lime juice. Blend until the mixture is smooth, adding more ice if needed to achieve the frozen texture.
Pour the pineapple mixture into the two chilled glasses, being careful not to mess up the coated rims. Garnish each glass with the blueberry cocktail picks. Serve the drinks immediately.
Notes
Since the pineapple gets grilled and then needs time to freeze, this is an excellent step to do ahead of time. When it’s time to serve your guests, you’ll have the pineapple ready and waiting in your freezer.