Cased just means that there is layer of glass on the outside over everything else, i.e., literally what it says. This layer may or may not be clear. The black speck is an impurity, probably a bit of carbon, that has got into the molten glass.
I suspect your vase is very modern production from China, elsewhere in the far East or Eastern Europe. These very large vases are turning up in "antique" centres all over the place in the UK. Some are better than others but all are new and factory produced. It often seems to be a case of never mind the quality, feel the width