Make the cupcakes: Preheat the oven to 350˚ F. Line a 12-cup muffin pan with foil or paper liners. Whisk the flour, salt, baking soda and baking powder in a small bowl. Combine the cocoa powder and boiling water in a medium bowl and whisk well. Let stand 5 minutes, then whisk in the sour cream and vanilla; set aside.
Beat the eggs and granulated sugar in a large bowl with a mixer on medium-high speed until light and slightly thickened, 2 to 3 minutes. Reduce the speed to medium low; beat in the vegetable oil until combined. Beat in the flour mixture in two batches, alternating with the cocoa mixture. Finish mixing with a rubber spatula. Divide among the prepared muffin cups. Bake until the tops spring back when pressed, 16 to 22 minutes. Transfer the pan to a rack and let cool 5 minutes, then remove the cupcakes to the rack to cool completely.
Make the fins: Line a baking sheet with parchment paper. Combine the white and black candy melts in a microwave-safe bowl. Microwave in 30-second intervals, stirring, until melted. Let the candy melts cool slightly, then transfer to a resealable plastic bag. Snip off a corner and pipe 12 fin shapes 1 1/2 to 2 inches high on the baking sheet. Refrigerate until hardened, about 10 minutes.
Make the frosting: Beat the butter and confectioners' sugar in a large bowl with a mixer on medium speed until combined. Add the vanilla; beat on medium high until creamy, 3 minutes. Add the milk and beat until fluffy, about 1 more minute. Tint with blue gel food coloring until the frosting is swirled.
Spread the frosting on the cupcakes and top each with a candy melt fin.