To make the donuts: preheat the oven to 375 degrees F. Coat a 12-cavity donut pan with cooking spray and set aside.
Whisk together the flour, sugar, salt, baking powder and baking soda in a large bowl. Whisk together the egg, buttermilk, oil, vanilla and water in a medium bowl. Whisk the wet mixture into the dry mixture and stir to combine. Fill a piping bag with the batter and pipe the batter into the donut pans, filling each cavity halfway. This could get a little messy.
Bake until a toothpick inserted into a donut comes out clean, about 12 minutes. Cool in the pan for 5 minutes. Remove to a rack and cool completely.
To decorate the donuts, dip them halfway into the glaze and then allow excess glaze to drip off. Sprinkle the tops with sprinkles and enjoy.
Blood Orange Glaze:
Combine the confectioners' sugar, honey and 4 tablespoons of the juice in a bowl. Add more juice little by little until the mixture is spreadable (you might not need the full remaining tablespoon). It should be quite thick yet spreadable.
Put the rhubarb, granulated sugar and 1/2 cup water into a small pot on a stovetop. Bring to a boil, then reduce to a simmer. Cook until the sugar is dissolved and the rhubarb is tender, about 10 minutes. Remove from the heat and allow to cool slightly. Pour into a blender and puree until smooth, then transfer to a bowl. Add confectioners' sugar as needed for taste and consistency.
Put the blueberries, granulated sugar and 1/4 cup water into a small pot on a stovetop and cook over medium heat. Bring to a boil then reduce to a simmer. Simmer until the berries have burst and the sugar is dissolved, about 10 minutes. Remove from the heat and allow to cool slightly. Pour into a blender and puree until smooth. Pour into a small bowl, straining if necessary, then add the confectioners' sugar. Whisk to combine, adding more confectioners' sugar as needed for taste and consistency.