Xavier, I know a number of Tea Traders have done just that, @iheartteas for sure. @keenteathyme too I think. I think Rachel created all sorts of teas there.
There are several companies that have a similar service. A couple of them are: Design a Tea and Ovation Teas. There are also companies like Mad Pots of Tea and Blends for Friends that will blend a tea for you based upon your likes/dislikes and/or personality profile that you submit to them.
I used to flavor/blend my own teas and operate my own tea business, but decided a couple of years ago that it was time to close it.