Depends on the region.
In the Rhineland/Limburg/Liège, it's made with no spots of fat in it, and 4th quarter pig meat (dark meat, meat was traditionally split up like this: the 1st quarter was for the noble and royalty, the 2nd quarter was for the clerus, the high ranking church officials, the third quarter was for the bourgeoisie, the rich commons, the 4th quarter was for the soldiers, and what remained, was the 5th quarter or the tripperies, the meat that isn't used by the butcher, and that's what the regular workers got), together with some bacon which is the source of fat. So it's not 5th quarter meat like in other regions, and there is no addition of molten fat, like in other regions. The spices are proprietary mixtures comprised of a lot of spices (spices have always been very present in the heart land of the empire of Charlemagne, which is due to the fact that Charlemagne made it mandatory for every church to have a garden with spices and herbs, originally mainly for medicinal purposes. Most old churches in the region still have such a garden.)
Here you can see an example of how it's made: http://www.een.be/programmas/dagelijkse-kost/bloedworst
That butcher is from Lanaken, which is Belgian Limburg, close to the Dutch and German borders and just north of Liège. He also uses buckwheat, not bread crumb.
In France, it's made in a different fashion, with boiled fat, bread crumb, onions and 5th quarter meat, cream and just pepper and salt, which results in black pudding with white spots of fat inside, the most common kind, that is found everywhere.
Here you can see an example of how it's made in Mortagne in the Vosges in France (region in the East of France, near Germany, South of Belgium): https://www.youtube.com/watch?v=HMqfLjmfyXI