Daniel Deronda is a deep blue-purple which would look stunning with a coral rose. Mine bloomed for the first time last year and wasn't without at least one flower from late May until frost. It's a type 2 prune which I don't always have great luck with, but this one is a stunning plant.
Rhapsody is another medium dark type 2 blue-purple which I've read in several places has a very long bloom period, though I don't grow it.

Also check out BLUE ANGEL 'Blekitny Aniol'
'Emilia Plater'
'Prince Charles'

I don't know how large your obelisk is, but be aware of the size of the clematis you decide on so that it is suited to the space available.

You can look up clematis on the Clematis on the Web website linked below, but in my experience, the color of the photos may be misleading, so read the color descriptions which are probably more accurate. Blue is a difficult color to photograph accurately.

Here is a link that might be useful: Clematis on the Web search page