Right on cue, Eddie Ross has further answered my question about my black pitcher. Silly me, I was trying just a little too hard. A simple link on his blog to the ebay listings of "wedgewood basalt" produced 68 results that look a lot like my little pitcher (and the ones in this picture), and a quick Google search found this. Mystery solved!
And by the way, I love how he spray painted that once-ugly urn with matte black paint to blend with the rest of his black Jasperware! What a great idea to tide me over until we can splurge on a few more pieces.