If your coffee liqueur is on the sweet side, then add six drops of vanilla bitters to help the balance and complexity. However, with a good coffee liqueur...
The citrus forward New Amsterdam gin (it's what I had open on hand) works surprisingly well with Mr. Black. I tend not to like citrus notes in my coffee, but as an iced cocktail it's quite satisfying. The New Amsterdam adds just a touch of herbal notes. Looking forward to trying this with Goldie's Gin