Book  (Oct 2001)  Page(s) 130-131.  Includes photo(s).
 
Clematis 'Blue Belle'
Viticella Group
(C. viticella 'Blue Belle')
Ernest Markham (England) mid-1920s
Temporarily lost to cultivation until re-introduced by Raymond Evison of Guernsey Clematis Nursery in the 1980s from a clone found in Canada.
... single, partially nodding, fully rounded, deep violet-blue flowers... six tepals... yellow anthers...