Are you sure?
Okay. Use responsibly.
Unsweetened. An ounce (one cube) or so per stockpot. This brand from the supermarket is okay, but we like the better-quality bulk stuff we can get at the co-op or specialty stores. Just toss it in when you add the tomato paste.
I have also used some unsweetened powdered cocoa that was so dark that no light could escape its surface, and that worked well. You want that wholesome bitterness to interact with the acid ingredients, meat and fats.
It adds richness, complexity, and authenticity to the chili. Chiles, beans and chocolate are traditional partners in Mayan cuisine.