Make the infused sugar: Combine the blood orange zest and saffron threads with the granulated sugar and use your hands to rub the zest and saffron into the sugar until it is fragrant and slightly colored by the zest and saffron. Set aside until needed.
Preheat the oven to 350 F. Grease a 10-inch cake pan. Dust the turbinado sugar over the bottom of the greased pan. Arrange the orange slices in circles over the sugar, pressing them closely together.
Make the cake batter: Whisk flour, infused sugar, baking powder, baking soda and salt in a large bowl.
Add eggs, buttermilk, orange blossom water and butter; mix until no dry spots remain (a few lumps are OK; do not overmix).
Pour batter over oranges in pan and bake until top is golden brown and cake pulls away from sides of pan, 35-40 minutes. Transfer to a wire rack and let cool 5 minutes before inverting and turning out onto a rack or a large plate.
Serve slices with labne and saffron honey.