This coffee-flavoured tequila sour is more brown than "Negra" (black) but is a tasty riff on a Margarita. How much agave syrup you should use depends on...
Comments about this being too sour make my head spin, I found this too sweet, and that was with Luxardo Espresso, a much dryer coffee liqueur. That said, this is a delightful, delicious Margarita variant and I really dig it. 2-3 barspoons of agave syrup is what I'll use going forward.
Agreed, as do other commentators. Hence, I have reworked the recipe to better balance. However, this is dramatically affected by your choice of coffee liqueur.
I make mine with equal amounts (1oz) each of tequila, lime juice, and coffee liqueur), but bumping up the tequila sounds like a good idea. May I suggest a light sprinkling of Taijin over the top of the drink in the glass, it adds a nice bit of complexity with a wee touch of heat.
This is a pretty genius concoction. Depending on the coffee liqueur you use, you can make it even blacker by topping with additional coffee (filter or even espresso). Great as a hangover cure :)