Preheat the oven to 350 degrees and line a cupcake pan with 4 liners.
In a mixing bowl, whisk together the flour, granulated sugar, baking powder, and salt.
Add the softened butter, egg, blood orange juice, blood orange zest, and milk to the dry ingredients. Beat until the batter is smooth and no lumps remain.
Divide the batter evenly among the 4 liners, filling each about two-thirds full.
Bake for 18 to 20 minutes, until a toothpick inserted in the center comes out clean. Let the cupcakes cool completely in the pan.
While the cupcakes cool, make the buttercream. Beat the softened butter in a clean bowl until creamy. Gradually add the powdered sugar and beat until light. Scrape in the vanilla bean seeds and add the milk. Beat again until the frosting reaches a spreadable consistency.
Pipe or spread the vanilla bean buttercream on the cooled cupcakes. Top each with one blood orange slice.