They look purple or lavender because they are.

Periwinkle blue is lavender toned blue.

I have yet to see a true blue Clematis.

There is a recent thread on the subject here somewhere.

Rosethyme, Blue Angle is a silvery blue that gets bleached out to almost a silvery white in my zone. Prince Charles is a pretty light bluish colored clematis that can also bleach out. In fact, the paler the color of the flower, the more that affect the bleaching action the sun can have on them. It's not that the darker ones don't bleach out but since they are more highly colored to begin with, it takes more sun to get them to a lighter color.

The others I have no experience with but there are no real blue clematis IMO. They all have some undertones of purple or red in them, it is just a matter of how much and how much you can tolerate. Photos are also notoriously bad since the flower color can depend a lot on age of bloom, how much sunlight it has grown in, the soil conditions it is grown under and the latitude the plant is grown in.

Thank you. I was thinking that's probably the case, like there are really no true red Clematis... always a bit of mauve or purple to the coloring. Maybe I should just get Mrs P B Truax since it is supposed to be a really good one.

Dorothy Walton is more lavender but the silvery colored star-shaped blooms are really pretty. Guess we just want them all. :)

Yes, we do want them all. :)