The habit of putting milk in tea reportedly started in France. Madame de Sévigné described how Madame de la Sablière launched the fashion:
Madame de Ia Sahlière took her tea with milk, as she told me the other day, because it was to her taste.
The Chinese black teas imported into colonial Boston surely would have tasted better with milk. The milk would have smoothed out the rough edges of the teas that were already stale by the time they made their years-long journey from Canton via London.
Much of the tea produced today in India, Sri Lanka, and Kenya is manufactured to be drunk with the addition of milk. Milk complements a malty Assam or full-bodied Sri Lankan black tea, but cream can sometimes mask the taste of quality black teas. It’s too heavy.