A quick suggestion Ceanothus or Buddleia.

I think that you'll struggle with pure blue but how about lilac. as you probably know there are more perrenials etc. that are blue

The other alternative is to plant something with glaucous blue foliage. There are several very good blue conifers. We have a very knowledgeable member on conifers on the site (bluespruce) and you would be able to contact him if you joined us

there is the caryopteris, they can have a blue flower as well as the already mentioned ceanothus which come in a range of blue shades.