How to Convert Blueberry Muffin Recipe to Vanilla Muffin Recipe
Blueberries are mostly water (~74%) and your recipe calls for a pint of them. Blueberries are roughly spherical, so they will pack into a container with a density of around 64%.

The recipe comes from a site in the US, so a pint is, presumably, ~473ml. One thing I don't know is how much liquid blueberries transfer to the batter while they are cooking. If they transfer somewhere between 25% and 75% of their water content, then you are looking to replace between 56ml and 168ml of liquid (i.e. roughly between a quarter and three quarters of a US cup).

Since there is already milk in the recipe, you might just as well add extra milk.