Black sea glass can be very misleading. When found on the beach, the piece often looks just like another stone on the shore. It's not until the piece is held to sunlight that the opaqueness is visible.
It is only then, the the beach comber will also discover the true color of the piece. Black glass is often not actually black, but a dark hue of another color. Most often the "black" glass is actually found to be a dark purple, deep green or brown. The piece will likely be quite thick, which helps in concealing the true color of the glass.
This type of glass is very old and not commonly found on the west coast of the United States of America.