I can only think of Agapanthus, Nigella, love-in-a-mist, Scabiosa, Delphinium & Lupin. Bg. also your florist can tell you what is available, to be honest most flowers can be got hold of if you really want them. Good luck

Along with those above are Corn flowers, Lavender and Nepeta, also HT Roses Blue Moon.

Not forgetting Camassia.....

If you are planning to make the bouquet etc yourself, try experimenting before the event, with white flowers. Stand them in water that has had blue food colouring added to the water. Carnations are very good for adopting a new colour.

Welcome to Goy. Lisianthus. The double ones look like roses and they have long stems. You should be able to get them in most florists but if not, there are detailed growing and care instructions on the following link. See them on this link.There is a list of stokists in UK too. http://www.dontveter.com/howtogrow/eustgran.html

Have you seen the new member Erkabobby who shows blue Alstromeria. Perfect Blue. You could ask your florist if this will be available and if they cannot source it, there are specialist growers in uk who can supply plants for you to grow your own. .