Delphiniums come in a few colours most commonly in a family of blues and purples from a darker navy to a lighter mint blue. Blue flowers are a great addition to any wedding as they also fulfil that traditional "something blue" requirement. Having the same flowers in several shades is a great idea for a floral arrangement, which gives it uniformity and slight variation. It also pairs great with navy for bridesmaid dresses which can be universally flattering.
For those who prefer a monochrome colour palette but want to move away from black and white, a design in tones of blue and navy can be seen as quite unique. And for those who want their wedding invitations to have a bit more bling, foiling the names in gold to offset the navy would be a great contrast.