Chuck, I still dig in Washington and we get black shelled clams all the time. They are usually older larger clams and are called "mossbacks" locally.  They are usually found on the beaches that don't get dug as often due to where they are located and where fish and game decides to open. The beaches that are open the most and get dug hard have the least mossbacks.  We harvest 2.5 to 3 million clams per year here between the Columbia River and Moclips. That's roughly 60 to 70 miles of beach.  On weekend digs in the spring, we get over 1,000 people per mile of beach. So unless a beach is mostly closed during the year, most clams don't live long enough to turn black shelled.  But I just dug last week in the Moclips area and got a limit of all black shelled clams.

http://wdfw.wa.gov/fishing/shellfish/razorclams/graphics/crowd_dig.jpg