can I substitute milk for buttermilk in blueberry muffins?

I've also seen plain yogurt used, in case you don't have lemon or white vinegar.

The best choice IMO is what Kenzi suggested. Alternately if the recipe calls for buttermilk and baking soda you could make it with regular milk and baking powder. 1 teaspoon baking powder has the same leavening action as 1/4 teaspoon baking soda.

Do you have a lemon or white vinegar handy? Just make your own buttermilk by stirring in one tablespoon of lemon juice per cup of milk and letting it sit for 10 minutes.