If the finish is really shellac, it will come off easily with denatured alcohol. If alcohol doesn't work, it ain't shellac!
Use superfine 0000 steel wool dipped in alcohol and rub it over the finish. The alcohol will dissolve the finish and the steel wool will strip off the black finish. The beauty of this process is that you can strip and refinish in one step if you don't rub too hard. There will be enough shellac dissolved in the alcohol so that when the alcohol dries, the shellac will be deposited on the surface as a "new" finish. It will take some practice.
Use lots of ventilation and keep away from flames as the alcohol is very flammable. It will dry fast.