Saute onions, bell pepper, garlic, and jalapeno in oil in a 8-quart heavy pot over moderately high heat, stirring, until softened, about 5 minutes. Add chili powder, cumin, and salt and cook, stirring, 1 minute. Add tomatoes with juice and zucchini and simmer, partially covered, stirring occasionally, 15 minutes. Stir in beans and chocolate and simmer, stirring occasionally, 5 minutes. Stir in cilantro.