Infuse the tequila with the butterfly pea flower for 5-10 minutes, and then drain.
Fill glass with ice cubes.
Pour the tequila and simple syrup into the glass.
Stir together in a separate container the grapefruit juice and seltzer.
Once ready to serve, top off the cocktail with juice mixture. Garnish with a charred grapefruit slice. You can do this simply by either putting the citrus slices on the grill, or you can use a kitchen torch. I find that the easiest and fastest.
Watch that witch craft happen as the cocktail turns from blue to shades of pink and purple.