[Update: July 16, 2011] This article was originally about huacatay, a Peruvian herb commonly called black mint. The herb you see above was sold to me as huacatay, and an insufficient amount of research led me to believe it was in fact that herb. SE'r Janel pointed out that the herbs above are in fact a variety of peppermint, also called black mint. This article has been revised accordingly to a discussion of black mint, not huacatay. Thank you, Janel, and the rest of the SE community for keeping me on my toes—and within my facts!
There's loads of mint varieties out there. We're familiar with some of them: peppermint, spearmint, apple mint. Others are more obscure, oddities like horse mint, corn mint, marsh mint. This is one of my favorites. Meet the curiously strong black mint. It's the mint you never knew you needed.
No mint I know has the fresh, clean aroma of granny smith apples and a deep, nose-clearing bite almost akin to cilantro. Black mint is sharper and mintier than garden-variety mint, cold and clean without all that fussy floral stuff on the side. It's a mint-lover's mint, the Chuck Norris of refreshing herbs.
Black mint's cool, clean quality is especially useful in chilled desserts that demand a sharp effervescent quality, like ice cream, sorbet, cheesecake, and cream pies. Sometimes even abundant quantities of mint can get washed out by dairy and sugar, tasting more "minty" than of actual mint. These are cases for black mint, which can turn a plain-Jane cold treat into a "holy alpine aromas, Batman!" sort of experience.
Cocktails are another calling for the herb. It's perfect paired with an especially sharp ginger beer in a Moscow mule (or better yet, a gin-gin mule. Its pungency can stand up well to strong liquors, though I'd stick to clear ones. Dark, rich whiskeys like bourbon deserve the more floral character lacking in black mint.
Black mint is also especially suited to savory applications. Its pungency pairs well with black pepper in pasta, or with smoky roasted red peppers in ajvar. And then of course there's the whole world of Peruvian cooking, one of the world's first—and most fascinating—fusion cuisines. There they blend an herb called huacatay with fresh cheese, milk or cream, aji chiles, nuts, and even stale crackers. The light but powerful sauce is ladled with a heavy hand on roast chicken, vegetables, or potatoes (that other Peruvian specialty). Black mint isn't huacatay, but its sharper, cleaner flavor makes it a decent substitute.
Fresh black mint isn't all that common, but is well worth seeking out. Farmers' markets are your best bet. The leaves are thin with a dark, almost purple tinge (but by no means as distinctive as their name suggests), with pronounced ridges. Black mint can also be identified by its clean, slightly sour apple aroma and, if you can pluck a sample, a deep, slightly bitter minty flavor. If you can't find fresh, frozen leaves are sometimes available. Failing these options, you could always combine plain mint and cilantro in a 3 to 1 ratio for a rough approximation.
Now to you, Serious Eaters. Had any chance encounters with black mint? Let's hear them—and your best black mint Chuck Norris jokes.