In a small bowl, mix together black pepper, paprika, chili powder, celery salt, cayenne pepper, garlic powder, and mustard powder. Rub mixture over the surface of the meat. Place roast in a slow cooker, and add 1/2 cup water.
Cover, and cook on Low for 8 to 10 hours. When meat is tender and well done, transfer roast to a serving platter. Skim fat from juices, and strain; serve with meat.