When potatoes are fully cooked, remove from oven and let cool for 10 minutes (leave oven on). They will still be hot, so using a towel or potholder, cut each potato in half. Hollow out each side, emptying contents into a large bowl. Add butter, sour cream, blue cheese, milk, salt, pepper, and garlic powder. Beat with an electric mixer until fluffy. Then spoon whipped potato mixture back into the potato skin shells.