I bought a Multi Blue Clematis earlier this year as well as Clematis 'Hagley'. The one which is meant to be Multi Blue is very disappointing, and although this flower is past it's best, it certainly wasn't the flower it was supposed to be.
Any ideas on what this is from the picture would be helpful.
I think this is just a recently planted clematis struggling a bit, due to a combination of factors. It will be concentrating on growing it's roots in the first year, plus the weather has been grim. The majority of clematis like to have their roots protected from the sun, but they like a bit of sun on their faces. You may well have some better blooms later in the season, especially if the weather picks up, and by next year with proper care your clematis should have more than doubled in size and will be blooming it's socks off. You'll find a bit of guidance here http://www.bbc.co.uk/gardening/plants/plant_finder/plant_pages/6788.shtml
How many blooms have you had from the Multi Blue? Have they all looked like this?
(Just a thought, what were they thinking of, calling it Multi Blue? So unimaginative. It looks a very pretty clematis - they make it sound like a pack of socks in M&S)
Having had a closer look at the picture, (with my glasses on) the leaf nearest the flower looks as if it could have had a bit of aphid damage, or even might have been caught in the spray-drift from some weedkiller - is that possible (it happens to the best of us - a sudden gust of wind - and it has been windy this year) or have you seen aphids/greenfly on it?
My first impression was that it was a perennial sweet pea, and not a clematis at all!