Low growing Campanulas make a nice blue carpet - in early summer, anyway. Lobelia if you don't mind that they're annuals? They sometimes self sow for next year.

most of the bugles have blue flowers. Brunnera, but perhaps too tall, anenome blanda and the de caen type also. there are small veronica's that might fit the bill, depends on how much sun they will get.

I'll have think.

Big choice of blue and bluey-mauve perennial geraniums too.

Blue & purple pansies

I'd go with Inverglen's geraniums, they spread nicely and are trouble free. Johnson/s blue, Brookside and Hocus Pocus are all lovely, the latter prefers shade. You could put the anemone blanda in nearly the same place as by the time the geraniums get going in the spring the anemones will have finished.

I got Johnstons blue geranium and veronica today plus a white Tree Peony.

Still to get white plants for either side of the blues.