I've occasionally found bits of tea leaves in the bottom of the cup. Ridiculously small though so it doesn't rule out the possibility of tea powder. You're absolutely right about it being a guarded secret though. I asked one of their employees what brand their tea is.
"Do you know what brand of tea you use? Like, is it written on the bag or anything?"
"Uhhh. Our supplier gives it to us." "So there's one supplier for all of Tapioca Express?" "Uhhh, yeah, that's right."
I'm pretty sure there's not one supplier for the whole chain. Considering first that I read somebody's response in a forum to the end that an individual store had changed their supplier so the tapioca was too hard.
So assuming it's a powder, is it a true tea powder, or is it more like the powder they use for Honeydew Slushes? Also, I wonder what sort of sweetener they use. Unprocessed sugar, white sugar, pre-made syrup, flavored sugar? The possibilities are manifold.
Also, somebody mentioned Yunnan tea or Duan Hong, forgot the name of it, but I'm interested in what the differences are between that between say Keemun and Assam and also where a good quarter pound can be found for under $8.