Le boudin noir is a traditional sausage belonging to the French food category la charcuterie.  It is usually made from fresh pig's blood (le sang de porc) and pig's fat (la graisse de porc) and can have additional ingredients such as chopped onion.  It can be served in many different ways: in the oven, in a pan, in pies.  It is often served with cooked apples. 

There are some variations, for example, le boudin noir can also include cooked pig's head and neck.  Sometimes, goat's blood or sheep's blood will be used.

If you want to eat lots of this nutritious sausage (it is rich in iron) then you should visit the French town of Conand where they hold a yearly festival dedicated to blood sausage.  It is called la Fête du Boudin.