There are no US guidelines for eliquid.
Make your own - I think Perfumer's Apprentice sells a Black Tea flavoring.
These are flavorings, not eliquids
Black Tea, Chai, Earl Grey, Green Tea
Perfumers Apprentice - Tea, Soda and Liqueur Flavors
For a simple way, go to Vaporbomb.com and buy plain unflavored PG/VG in your nicotine level and a couple of small blank bottles and something to measure with like 1ml pipettes or small syringe - figure out how much flavoring you like. And you may need a bit of Eliquid sweetener which for tea would be a Splendex diluted first with PG because it's too strong.
Or for the more correct way, go to OneStopDIYShop and buy a kit for making juice and use an eliquid calculator.
If you want to know if anyone sells a black tea eliquid, go to Google and search for black tea eliquid or black tea ejuice but some crappy vendors may just relabel the Dekang. If you find a shop with it that offers PG/VG ratios then they make it themself. But you can probably find some real juice makers who offer Black Tea flavor.
Caveat - it may not taste the same. There are at least a dozen brands of flavorings and I would bet Dekang in China uses none of the ones we know about - they probably use a Chinese brand of flavoring.