Black lacquer is used over both sabi, (jinoko or tonoko/lacquer mix) and mugiurushi, (flour/lacquer mix) as a layer to give you a smooth finish. Without a middle layer of lacquer it is very difficult to get a smooth enough finish to successfully apply a top layer of lacquer and then metal. Applying a layer of black also hardens up the sabi which is a much nicer surface to sand and shape compared to an uncoated, ‘naked’ layer of sabi. Sabi has a rough surface until it is coated. It is made from jinoko which is a non-plastic type clay so there will always be some grit till it is coated. If you use tonoko to make your sabi it will still have some grit although less than jinoko. There are many types oflacquer, top, middle, and what I call bottom. Middle is not as hard as top lacquer. The lacquer I link to above is a top layer which is a better quality than a middle black.