Finally got around to trying this in a London Fog today. Treating the tea so harshly (ha!) negates the overwhelming “Earl Grey”-ness, while the blackberry-vanilla blends in with the vanilla syrup and milk to make an interesting drink. Although, towards the end of my cup, I got a weird acidic taste; not quite sure if it was over-steeped tea (though I only steeped this for 3 minutes and used my Ingenuitea to brew it) or the milk reacting with the citrus in the EG, or something else altogether.
In case you want it, here’s a standard, very malleable recipe for a London Fog:
I have an immersion frother like they sell at Ikea and World Market
I generally make 12 ounces of tea, 4 ounces of milk, and use 3 pumps of Torani Vanilla and a little less than 1 full pump of Torani Hazelnut when I make a London Fog at home.