The United States is not a place you find a lot of blood sausages. To be honest, boudin rouge in Louisiana is the only one I can think of in the whole country, and even that is a very niche product. It is difficult to find any commercial producers of it outside of Lafourche Parish.
Not so in Spain! It seems that every region of Spain has it's own morcilla (or two). Asturias is no different. In Asturias it is common to find two morcilla sausages commonly used.
The most common, and the star of Fabada, is the Morcilla Asturiana. This is a sausage made from pork fat, blood, garlic, and massive quantities of caramalized onions. Seasoned with aniseed, clove, cinnamon, and nutmeg, this is a pretty exotic sausage by Asturian standards. Once assembled and stuffed, the sausages are cold smoked for weeks. Traditionally this was done by hanging the sausage links near the open, unvented cooking fire, and later by hanging them in the chimney. These days it is done commercially in curing rooms, or at home in smokers quite like what you might find in an American barbecue enthusiasts back yard.
More commonly found on Asturian parrilladas (mixed grill platters for sharing) than in stews, this morcilla from neighboring León is also wildly popular. Less exotically spiced, and often with a small structural addition of breadcrumbs in place of some of the onion, this sausage is usually consumed fresh. It is more common in León to find it uncased in glass jars than cased as sausage links. That link form though is what Asturians favor, and a blistered matachana splitting open on a plate of other grilled sausages, ribs, and filets, is the sign that you are in a great place to eat.