The common denominator is the bergamot trick, a type of adulteration which is older than Earl Grey itself. Here it is described in an 1824 issue of
The Kaleidoscope, for improving the flavor of green tea:
To render Tea at 5s. a pound equal to Tea at 12s. -
The cheapest and most expensive teas are all the leaves of the same tree, at least they should be so, and if there were no sloe-leaves nor privet-leaves, they would be so. The high flavour, therefore, of some of the sorts of tea, and the want of flavour in others, must arise from the manner of preparing them, and must be in some measure artificial. It follows, that if we can discover any fine flavoured substance, and add it to the tea in a proper manner, so as to make it agree and harmonise with the original flavour, we shall be able to improve low-priced and flavourless tea, into a high priced article of fine flavour. The flavouring substance found to agree best with the original flavour of tea, is the oil of bergamot; by the proper management of which you may produce from the cheapest teas the finest flavoured bloom, hyson, gunpowder, and cowslip. There are two ways of managing the bergamot. Purchase at the perfumers some of the perfumed pieces of wood, which they call bergamot fruit. Keep one such piece in your canister, and it will flavour the tea in the same way as a Tonquin bean flavours snuff. If the canister be a small one, the flavour perhaps would be too strong; in that case you may chip the bergamot fruit in pieces, and put only a little bit among your tea. Or procure a small phial of the oil of bergamot; take some of the smallest of your tea and add it to a few drops of the oil, till you form a sort of paste, which is to be carefully mixed with the whole tea, in proportion to its quantity and the degree of flavour you like best. If you make the flavour too strong, you have always on easy remedy, namely, by adding more unfavoured tea. When it is thus improved, it is often sold at 18s. and a guinea, a pound. Cowslip tea has been as high as 32s.
(
https://books.google.com/books?id=1DAFAAAAQAAJ&pg=PA435)
I suspect this was a British effort to independently replicate "Canton Tea" that was scented before sale to the East India Company, not quite understanding how it was done in Guangdong.
However, it was already a decades-old practice by 1824 -- here is a description of the hustle in action from 1785 (along with an antique variant of MarshalN's
ABA trick), courtesy of John Trusler's novel,
Modern times: or the adventures of Gabriel Outcast:

- bergamot-trick-1785.jpg (810.44 KiB) Viewed 5186 times

- bergamot-trick-1785b.jpg (413.91 KiB) Viewed 5184 times
Much like with jasmine tea, eventually a preference developed for this flavor even when the deception was no longer a deception.
