Bring 2 cups water to a boil. Add 4 black tea bags, steep 5 minutes, then remove the bags. Let cool 15–20 minutes and refrigerate until well chilled.
In a large pot, bring 8 cups water to a rolling boil. Add 0.5 cup black tapioca pearls (quick-cooking), stir to prevent sticking, and cook until the pearls float and are tender with a chewy center, 20–30 minutes. Turn off the heat, cover, and let sit 20–30 minutes.
Meanwhile, combine 0.5 cup dark brown sugar and 0.25 cup water in a small saucepan. Bring to a simmer over medium heat, stirring until dissolved and lightly syrupy, 2–3 minutes; remove from heat.
Drain the pearls, rinse briefly under hot water, drain well, and stir them into the warm syrup to coat. Keep warm off the heat.
Divide the warm pearls and a few spoonfuls of syrup between two 16-ounce glasses. Add 2 cups ice cubes to fill each glass about two-thirds.
Pour the chilled tea over the ice, leaving room for milk. Taste and stir in more syrup from the pot to sweeten, as desired.
Top evenly with 1 cup whole milk. Stir (or cap and shake) until tan and well mixed. Serve immediately.