If you go on the RHS Plant 'Find A Plant' site (http://www.rhs.org.uk/plants/search-form) you can enter all that information in the drop-down boxes and it will give you pictures of the ones in that category. I'm sure that there are other sites that offer the same, just Google plant finder or plant chooser.

You could also use blue ceramic pots that will be blue all the year round. Most of the summer blue ones I can think of aren't really pot plants or are shorter or taller than your specifications!
Your garden could really take delphiniums in front of the fence if you've left enough ground - a bit of height would really make a difference.

Thank you steragram you must read our minds, the wife said use blue glazed pots, and I said we need height. At the moment may do what myron has said and investigate a bit more on rhs.

I know that it is sometimes classed as a bit of a weed but I really love Centaurea Montana, lovely blue flowers and lots of them!

My original clump of Delphiniums are 5ft in height W1nk, possibly too tall for what you want, I split the clump last year and the transplanted ones are also very tall but they are gorgeous...Hope you find something that pleases both of you..

Yes delphs are too tall for pots - I was just wondering if W1nk had left enough border soil to put some by the boundary.

I realise that you probably only did it recently but I'd be tempted to paint the fence especially in the seating area close to the shed - there are a couple of shades of blue that would compliment the blue of the shed - and would provide a contrasting background for the greenery.

Thats right Stera, that was my thought as some of the ones I transplanted last year are planted very close to our fence and it doesn't seem to have stopped them thriving, also those are protected by the fence so no need to stake....

Some of the bedding lobelias will be ideal, and some of the perennial salvias too.
Ceanothus is available as a prostrate form, might be a bit too pale but see what you think.
Lupin The Governor will give short-term drama but probably wouldn't be happy in a pot for long.

Thanks for your ideas everyone, will look into for best remedy.