It's very common to be served beans as a side dish when you go out for lunch in Mexico. Whether you're stopping into a little street stall for lunch or you're going to a friend of a friend's house for comida, Mexican beans recipes like this recipe for black bean soup are very often made and served as a 1st course for the meal, to tide you over until the main course arrives.
If you like beans and you like authentic Mexican food, try the recipe out, let me know if you like it, and leave me a comment to let me know if you'd like to see more authentic Mexican bean recipes like this in the future. This is a very easy recipe to try out, so it's a good one to experiment if you're new to authentic Mexican cuisine and you'd like to test the waters first (and wet your appetite!) before diving head first into a word of chiles, tortillas, and--yes--beans. Speak to you again soon, -Carlos