Wash the orange and blend it whole until smooth, with no large chunks remaining.
In a large bowl, whisk together the eggs, milk, oil, and vanilla. Stir in the blended orange.
Add the flour, sugar, and baking powder, and mix just until combined—do not over-mix.
Gently fold in the raisins.
Divide the batter evenly between 9 muffin cases.
Bake at 175°C (350℉) for about 22 minutes, or until a skewer inserted into the centre comes out clean.
Allow the muffins to cool slightly. They are best enjoyed while still warm.