Preheat the oven to 350 F (180 C)
Soak the tea bag in 1/2 cup of boiling water for 10 minutes whilst you melt the butter in a medium pot and remove from the heat. Beat the sugar and eggs into the butter, followed by the buttermilk and lemon zest. Remove the tea bag from the water, pour the brewed tea into the pot and stir together. Cut open the tea bag and stir the tea leaves into the pot as well. Add the flour, baking powder + soda, salt, poppy seeds (if using) and stir together until just combined. Stir in the blueberries.
Pour the batter into a greased and floured large loaf pan. Bake for 45-55 minutes, until a skewer inserted into the center of the cake comes out clean. Leave to cool for 10 minutes in the pan before turning out onto a wire rack.
Make the glaze by stirring together the powdered sugar and enough lemon juice to make a thick, pourable mixture (add the lemon juice slowly, stirring between additions). Pour over the warm loaf and leave to cool and set.