Siberian Iris 'Perry's Blue' (Iris Sibirica) is fabulous looking with its display of sky blue flowers adorned with delicate white markings and dark veining that can be admired from late spring to early summer.
It's hard to find a photo that does this plant justice. It's very tall and transparent and the leaves are a gorgeous blue/green colour. It doesn't mind a bit of shade. It would go at the back of the border.