Line a 9-inch square pan with aluminum foil and set aside.
Heat milk over medium heat until warm; add sugar. Bring to a rolling boil
over medium-high heat while stirring constantly with a wooden spoon. Add Marshmallow
Creme and butter. Return to a rolling boil for 5 1/2 minutes (start timing as
soon as the rolling boil resumes).
Cut cream cheese into small dice, and add it to the boiling mixture about
1 minute before the end of the boil. If brown flecks begin to appear in the
mixture, lower the heat a little.
Remove from heat and add vanilla chips and blueberries. Stir until creamy
and all chips are melted.
Stir in vanilla extract and butter flavoring or extract. Mix thoroughly.
Pour into prepared pan. Cool.
Remove from pan; remove foil, and cut into squares.