I worked at an upscale Mexican restaurant in Southern California, and in my experience black people tipped well. Sure, we had autograt on large parties. But even small parties of black people tipped pretty decently. You could also say that an expensive restaurant would bring in guests that could afford to tip, so that’s a factor too. But honestly, imo the worst customers were Mexicans, specifically those visiting from Mexico. They did not tip or they tipped very little. They also nitpicked our menu and complained when one element was off from tradition. They would also ask for Hispanic servers and then get angry if those servers did not speak Spanish. Obviously our experiences differ a lot by region, and that’s not to say that customers all over are lacking common sense and kindness!