A whole lot of clematis are pink or purple. Is lavender OK by your taste (there are several of those) or is the shade of blue-purple that is commonly known as blue in the horticultural world? When I plugged the word "double" into the text box on the search page for Clematis on the Web I got a list of 461 plants. I am sure that many of them aren't actually the typical multiple petaled large flowered hybrid (semidouble or one of the early flowering type 1). Of those that are similar to Josephine, many aren't commercially available in the US, but some that I've seen on nursery plant lists that aren't pink and purple:

Arctic Queen Evitwo (white)

Mary Rose viticella 'Flore Pleno' (deep red-purple)

Veronica's Choice (White to lavender)

Viennetta Evipoosix (white with purple stamens)

Also look at Avant Garde Evipo33 which while red with pink stamens and is quite small, might appeal to you.

I think Raymond Evison must really like double flowers since a lot of them seem to come out of his breeding program.