Preheat oven to 400 degrees. Line a muffin pan with cupcake liners.
Make streusel. Combine walnuts, flour, brown sugar, and oats in a medium bowl. Add butter and stir until well combined. Set aside.
Combine blood orange zest with sugar in a large bowl and rub zest with sugar until well combined. Add flour, baking powder, baking soda, salt, and cardamom.
Whisk together buttermilk, oil, blood orange juice, egg, and vanilla extract until well combined. Add to flour mixture, mixing just until combined.
Fill muffin pan about ¾ full. Top with crumbled streusel and bake muffins for 18-20 minutes or until a toothpick inserted in the center comes out clean. Remove muffins from pan and let cool completely on a wire rack.
Notes
If blood oranges aren't available, substitute with regular navel oranges or cara cara oranges.