I tend to have black tea in a flask. Something about the milk in it makes it taste funny to me. Get used to your normal tea without milk in it.
even if you do like milk in tea or coffee just don’t put it in the flask, add it when you drink it. Darjeeling’s a good tea for drinking black from a flask though.
If you’re used to milk its such a flavour suppressor it can be difficult to get used to things that actually taste of stuff. – “white tea”, not tea with milk in it, but tea made from the same tea bushes but dried and processed differently, and not as oxidised, is a good tea for people who normally drink milk. Nice with a mint leaf in there too (keep a bag of fresh mint in the freezer and drop a few leaves in the flask) Strictly speaking white tea is a black tea rather than a green tea, if you get my drift, but its more of a beige-ish yellow in the cup.