"Do you want black tea?"
Translation:Chcete černý čaj?
"Dáš/dáte si černý čaj?" is definitely a very natural way of asking someone "Would you like black tea?", when you refer to actually drinking a cup. On the other hand, "Chceš/chcete černý čaj?" is a better question when someone wants to give/sell a pack of black tea leaves/bags.