I really like strong blue flowers and here we have four of the best, all available now.

Geranium 'Johnson's Blue' is an excellent ground cover plant with masses of large, dark-veined, lavender-blue flowers that fade to pearl grey as they mature. A tough, easy to grow plant.

Primula 'Noverna' has striking purple-blue drumstick shaped, very scented blooms, on tall stems from spring all through summer, often into September. 

Corydalis 'Blue Heron' is a stunning hardy perennial with electric blue flowers over a long period from spring to early-mid summer. A really eye-catching plant which is not at all fussy and is easy to grow.

Camassia esculenta is Wild Hyacinth and deserves to be better known. It's completely hardy and comes back year after year. Bees love the flowers and they're largely untroubled by slugs and snails. Their sturdy stems don't need staking and they make good cut flowers.