Mince garlic in the bowl of a food processor. Add black beans, 2 tablespoons reserved liquid, 2 tablespoons lemon juice, tahini, 1/2 teaspoon cumin, 1/2 teaspoon salt, and 1/8 teaspoon cayenne pepper; process until smooth, scraping down the sides as needed. Add additional seasoning and liquid to taste. Garnish with paprika and Greek olives.
