Need not necessarily be the ceramic magnets with black surrounds, I have several pairs with Alnico magnet bass units, and they have black surrounds. There was a change point from white surrounds to black surrounds fairly early on. The date will probably be on the Spendor Yahoo Newsgroup.
I also have a single pair of black surround ceramic magnet BC1's, and several BC1's with Alnico magnets and white surrounds.
Just thought, to confuse the situation a bit more! I have a pair of BC2's which have white surround bass units with ceramic magnets.