Blue bell, or, more properly, bluebell, is a name given to flowers that may or may not be closely related botanically, though they do have a few characteristics in common. First, as the name suggests, they're blue; second, they're bell-shaped. Three flower genera, especially, fit this description and are commonly referred to as bluebells: Hyancinthoides, Campanula and Mertensia.
As you might guess from the name, this bluebell genus is a member of the hyacinth family. Three species belong to Hyacinthoides: the common bluebell, Hyacinthoides non-scripta; the Spanish bluebell, Hyacinthoides hispanica; and the Italian bluebell, Hyacinthoides italica. Bluebells bloom in spring, filling woodlands with what looks like a sea of blue, while overhead, the leaves turn green.
Rotundifolia is the species of the genus Campanula commonly called bluebell. It is a flower of the bellflower family; indeed, campanula refers to bells. Other common names for the plant are bluebell-of-Scotland and harebell. In Scotland this plant has also been called witches' thimble. This flower grows with a lot of variations depending on habitat.
With its 40 or so species, the genus Mertensia belongs in the borage family. Some of these species include the Virginia bluebell, also called cowslip; the beautiful bluebell, known also as the Oregon lungwort; and the mountain bluebell. These all grow in the United States, springing up from meadows to mountains.
The bluebells belonging to the genus Hyancithoides, grown from bulbs. The woodland perennials put up stalks, each stalk putting out fragrant flowers with nectar that supplies both honeybees and butterflies with food. Campanula, another perennial, also produces fragrant flowers, though, unlike the spring-blooming Hyancithoides, Campanula arrive in late summer. The herbaceous Mertensia are also perennials, growing from seed.
Hyacinthoides species are European. About half the world's population of common bluebells is found in the British Isles, where they are protected. The Spanish bluebell is native to Spain and Portugal, but gardeners have introduced it to other places like England, where it has escaped to the wild.
The Italian bluebell is native to several places in the Mediterranean, including Spain, France and Italy. Campanula rotundifolia is found in North America and Europe; in Scotland, this flower is called bluebell. The different species of Mertensia are found in North America and Eurasia.
The common bluebell is often thought of as the national flower of Great Britain, where it is beloved. Massive blooming of the flower in spring creates what are known there as bluebell woods. Bluebell woods occur in old growth forests, or what is called "ancient woodland" in Great Britain.
Campanula rotundifolia has many associations with folklore, as evidenced by common names like "fairies' thimbles" and "dead men's bells." The flowers are the symbol of the Scottish MacDonald clan. The New World also has lore associated with Campanula rotundifolia; Northwest Native Americans thought picking them brought rain.