Place salt in a saucer. Moisten the rim of a glass with a grapefruit slice. Dip rim in salt. Combine juice and vodka with Campari or grenadine in the jar of a blender; blend. With the motor running, add ice until fully incorporated and slushy. Pour into prepared glass.
While this cocktail is refreshingly simple, the bite of salt adds intrigue, and Campari pairs effortlessly with the grapefruit for a flavor experiment worth repeating.
(Adapted for 2, along with other Campari recipes at http://www.aperiti.com/blogs/aperitivo/6453752-19-ways-to-drink-campari!)