Bituminous coals from West Virginia and the Appalachian basin make excellent blacksmithing coals.
Coal from the Sewell vein, Pocahontas vein, and others are known as "metallurgical coal" or "met coal" because of their high heat and low impurities such as sulfur and ash. Met coal is very good for blacksmith forging because it converts to coke leaving few impurities (clinkers) in the fire pot.
Coal from the Pittsburgh vein, Redstone vein, and others are called "steam coal" because they are ideal to fire a steam boiler (locomotive, power plant, etc.) You can forge with these coals, but you will have to contend with clinkers.
Some coal companies sell "washed coal". The wash process is actually a way to separate slate and other non-coal binders from the coal using controlled specific gravity to float the coal. Washed and sized coal burns hotter and cleaner than raw coal.
You can forge with either raw coal or washed coal.