In a small saucepan, heat milk just to the simmer. Off heat, add the Black Currant pyramid infusers, submerging them completely. Set mixture aside to cool. When cool, remove infusers, squeezing them to remove liquid.
Adjust rack to lower third of oven and preheat oven to 350 degrees. Grease and flour a 9-inch square cake pan.
Sift the flour, baking powder, and salt. Using an electric mixer at medium speed, cream the shortening and the sugar to blend thoroughly. Add the egg and vanilla and beat until light and fluffy, about 4 minutes. Add the flour mixture alternately with the steeped tea-flavored milk mixture in three or four additions.
Fold in blueberries and spoon batter into pan. Bake for 25 to 30 minutes or until cake springs back when lightly pressed in center. Cool on wire rack 10 minutes before inverting.