HI WendyS - black raspberries were identified a few years ago as being the highest scoring fruit in trials for anti-oxidant levels - able to clear free radicals and reduce incidence of cancer in rats - at 40% better than strawberry and over 10% better than blueberry, so why we don't see more I've no idea.
The varieties are either Rubus occidentalis or Rubus leucodermis and don't seem to be available at all in UK as canes.
Try calling Brogdale National Fruit Centre 01795 535286 and ask if they can point you at any supplier for gardeners of indeed if it is grown here for consumers.
Also try Barry Clarke at the Rubus collection at Romsey 07919 893 060 - he runs part of the Sir Harold Hillier Gardens and a website on rubus varieties:
http://www.rubusspecies.com/index.html and if anyone can tell you they are available, he should know.
Or just grow one of the others.....