A picture would help. There are more than one that fits your description.

I agree that a picture would help out but from your description of the clematis the first thing that pops into my head as a possibility is perhaps Blue Angel (Blekitny Aniol). In my garden it gets bleached out very easily and almost looks silvery gray. It is one that I can never take a picture of and get to show the true color of the flower when it first opens.

I think it's a Miss Bateman cuz my records show that I planted one there a couple of years ago. I just didn't go back two years, or three. I still don't think it's white, and the info I found thru googling shows a little greener tinge to it. But the center of the flower looks real similar.

Thanks everyone for trying!