In a small mixing bowl, beat cream cheese and sugar until smooth.
Add egg and lemon zest, and beat until smooth.
Fold in the blueberries. Set aside.
In a medium mixing bowl, beat butter and sugar until fluffy.
Add eggs, one at a time, beating well after each addition.
Beat in milk and vanilla.
In a small bowl, combine flour and baking powder.
Add the flour mixture to the creamed mixture, and beat until just combined.
Line 16 regular-sized muffin cups with paper liners.
Fill each cup 1/3 full with the batter.
Drop a rounded tablespoonful of the blueberry-cream cheese mixture on top of each muffin. Prepare streusel.