Preheat the oven to 375F.First make the streusel topping. Mix together the flour and brown sugar. Cut in the butter until crumbly. Set aside.In a bowl, mix together the flour, sugar, xanthan gum, salt, and baking powder.Make a "well" in the dry mixture and pour the oil, milk, egg, and vanilla into it. Whisk the wet ingredients together, and then mix it all together.Stir in the blueberries.Fill muffin cups lined with paper liners about ½ full of the batter. Top with the streusel crumb topping.Bake for 22-28 minutes, or until a toothpick comes out mostly clean, with maybe a few little crumbs.