How to Make It
Stir together 4 cups of the water and 1/4 cup of the flowers in a large bowl. Let steep until water is a deep blue color, about 5 minutes. Strain; discard flowers. Pour water into an ice-cube tray. Freeze until solid, at least 8 hours.
Grind remaining 2 tablespoons flowers in a spice grinder until finely ground, about 3 minutes. Stir together salt, sugar, and ground flowers in a small bowl; transfer to a plate.
Pour remaining 1/4 cup water into a shallow dish. Dip rim of glass in water, and then into ground flower mixture, coating rim of glass. Fill glass half full with blue ice cubes; place grapefruit peel in glass.
Combine grapefruit juice, tequila, lemon juice, lime juice, and honey in a cocktail shaker; shake until fully combined, about 15 seconds. Pour into glass over ice; top with club soda. Sprinkle with flowers.