Blue boy is a large integrifolia clematis with blue bells. It doesn't climb though, so you would have to tie it or push the branches into the arborvitae. The species viticella has a nodding blue bell and it climbs and would have no trouble scaling a 12' arborvitae. Perle de Azure has a light blue nodding flower and gets very large and produces a lot of blooms and climbs. Pitcheri has a purple bell and climbs and gets about 10' tall for me. You can check out photos on clematis on the web.

Here is a link that might be useful: clematis on the web

You will probably find the biggest selection of climbing blue, bell-shaped flowers among the Atragene group - cultivars of C. alpina and macropetala. Although an early and somewhat short bloom period, they will easily climb to 12'.....perhaps quite a bit more. And although the flowers are not exactly bell-shaped, 'Harlow Carr' is an excellent choice. It is considered by many to offer the longest and heaviest bloom of any clematis and the flowers are a deep, dark blue that will show off beautifully against a gold conifer.

{{gwi:617856}}

Mine rapidly grew to 10-12' (second season) and is easily my favorite of the many clems I've grown.