Carioca bean" (feijão carioca in Portuguese), in fact that bean is not the most commonly eaten bean in Brazil's most famous city. That honor must go to the black bean, or the black turtle bean as it's sometimes called.
Brazil is a vast country, and although dried beans are a staple of the Brazilian diet everywhere in the country, the type of bean which is preferred is a regional choice. In some parts of the country white beans are standard, in others brown, or pinto-type beans are the norm. Only in Rio de Janeiro (the city and the state) is the day-to-day choice of bean likely to be the black bean.
When Brazilians anywhere in the country made feijoada (click here for more information), the use of black beans is obligatory, because this dish is particularly associated with Rio de Janeiro. In Rio itself, it is estimated that more than 75% of the population eats beans on any given day, and the vast majority of those beans will be black turtle beans, normally eaten with white rice.
Black beans are particularly appreciated throughout Latin America and the Caribbean, and it's impossible to imagine Mexican or Cuban cuisine without them. I find them to be one of the more flavorful beans, and one of the most suited to tempering with spices, herbs and citrus juices.
All dried beans are very healthy dietary choices, and the black bean is one of the most beneficial in the whole bean family. It is rated as being extremely high in levels of molybdenum, very high in folicin (vitamin B9), tryptophan and fiber, and high in protein, vitamin B1, magnesium, manganese, iron and phosphorus.
Besides its use as a daily staple, the back bean is used in soups, stews and, of course, in the national dish feijoada. (Click here for a bean soup recipe that can easily be made with black beans, and here for feijoada).
North Americans would do well, diet-wise to imitate the bean-eating habits of Brazilians. Dried beans are inexpensive, easy to preserve and store, versatile in cuisine, healthy and delicious. What more could one want from a food?