Depends on how much, I'd say. If you're using tea basically as the brew water, you could even add the leaves to the mash, or at flameout in the boil. But the flavor may dissipate or change during fermentation. It might be an interesting experiment. But if you want to have a black tea flavor in the final product, the most reliable way to do that would be to add it at bottling. Make sure to account for the volume it will take; tea isn't the most concentrated of flavors. To my mind, do this if you're only adding a bit of tea.
If adding more, I would just add leaves to the fermentation vessel after primary fermentation, much as a dry hop. Cold-brewing tea makes for a pretty similar result as that brewed at high temperatures. Just dump a little boiling water over them first; this will both sanitize them, and 'awaken' the leaves.