Blue must be unpopular with the more common of pollinators. After all, colorful flowers are designed to attract some sort of vector to exchange pollen. It seems that most pollinators like yellow. Red and pink (which is actually a tint of red) seem to be more appealing to hummingbirds. Butterflies seem to really like pink and orange. Many flowers that seem to be white actually use infrared to attract bees, or ultraviolet to attract nocturnal moths. Other white flowers rely on wind, which is not discriminating about color.
Of the primary (red, yellow and blue) and secondary (orange, green and purple) colors, the only color that is more uncommon than true blue is green. That is probably only because green flowers do not contrast much against green foliage, so would be harder to find. Many blue flowers, like thyme, lavender, blue potato bush and jacaranda, are closer to purple. Some blue and purple color is probably only incidental, and in conjunction with invisible (to humans) ultraviolet coloration or patterns.
Except for purplish blue jacaranda, there are no substantial trees to bloom blue. Empress tree is flashy, but is even more purplish. However, most ceanothus, including a few that can grow as small trees, are famous for their clear blue bloom. Plumbago, blue hibiscus, echium and rosemary are blue blooming shrubbery. Creeping rosemary is a nice groundcover. Hydrangea has the potential to bloom blue, but often turns purplish or pink because of soil alkalinity. Lilac and wisteria vine can be lavender, pink or white, as well as ice blue.
There are several perennial and annual salvias and lupines that bloom blue. Russian sage, catmint, carpet bugle, campanula, perennial statice and lily-of-the-Nile are some of the other familiar perennials for blue flowers. Delphinium should probably be more familiar. Grape hyacinth and various iris bloom briefly but spectacularly with some of the richest shades of blue. Even though there are not many blue flowers to choose from, there is quite a bit of variety.
Annual statice, aster, zinnia, bachelor button, nierembergia, nigella, pincushion flower, forget-me-not and cineraria are uncommon annuals that are enjoyed by those who crave blue. Petunia happens to be one of the most popular of warm season annuals that also happens to produce some excellent blues. Later, brightly colored pansy and primrose can be just as flashy as popular cool season annuals.