I've had it for years. it was supposed to be integrifolia, not!

Look at this link - http://www.clematis.hull.ac.uk... - that would be my guess - C. 'Blue Belle".

Thanks, that sure looks like it, unless it is Etoile Violette which has smaller flowers. Hard to judge what small means when you only have one!

I amend my guess; the flowers on BB have a small separation between them, the same is not true for yours. So I do think you're right - COTW says the flower sizes are the same - between 5-10 cm, roughly 1-2". Is that how big your flowers are?

Yup! But they do have separations.