It's definitely hard to tell what's happening as they steep, that's for SURE! Just fyi, when I suggested "wee tiny untasteable", I was thinking somewhere around .25% - .33% (maybe .5% max). 1 drop per ml is 'typically assumed' (whatever that is :::eyeroll:::, because it can be crazy different) to work out to around 2.75%- 3%...so 1 drop/2ml might be around 1.5%. And with FA fruit against FA Virginia, it might be a bit too much.
But your choice of Black Cherry gives us an idea...how about 2.5 Viriginia; .5% - .75% Cocoa, and .5%-.75% Black Cherry? That might be a major winner (or seriously disgusting, lol).
Well, we're off to give the Irish Lass a taste-vape!