I don't know how you take your coffee but if you're looking for an accurate strong black coffee flavor I recommend buying some Lorann coffee flavoring. You can throw it in a bottle of unflavored nic juice for true black coffee or you could just add a little to the underwhelming coffee juices you already have. ooor you could mix some in Caramel juice for a caramel macchiato... anything you like.
Coffee flavoring is one of those distinct flavors that you can really taste when you add it to anything. It's an excellent one to just have around, even if you're not interested in the full DIY route at all.
I have Lorann "Coffee" and "Keoke Coffee"- both taste... well, like they're boiled down coffee actually. And they cost me $1.50 each. I bought them at a local baking supply shop.
I drip/mix on the fly: if I'm vaping a chocolate cart and I want choco-coffee flavor I just use a little pipette to put a tiny drop of coffee on the atty. Rocks!
In premade- JC Espresso is more on the serious side, not a sweet coffee for sure. It's an acquired taste though.