Blood sausage is a sausage made primarily from animal blood — typically pig, cattle, or sheep — combined with fat, grains, onions, and spices, then encased and cooked. It appears in some form in most culinary traditions that raise livestock, as a way of using the whole animal.
Names and recipes vary by region and aren't fully interchangeable: British and Irish black pudding is bulked out with oatmeal or barley and served as part of a full breakfast; French boudin noir is finer-textured and often includes cream, onions, or apples; Spanish morcilla frequently adds rice; and Asian versions such as Korean soondae are typically stuffed with glass noodles rather than grain. The common thread is blood as the binding, coloring ingredient, cooked until it sets into a firm, dense texture.
Flavor is rich and earthy with a slightly mineral, iron-like note from the blood; black pepper, nutmeg, and allspice are common seasonings used to round it out. It's usually sliced and pan-fried or grilled, and served with mashed potatoes and fried eggs in the UK and Ireland, or in hearty bean stews such as fabada asturiana in Spain.