Many flowers are called “blue” when in fact they are usually more purple than blue. In nature you rarely find a true blue flower, and breeders find it difficult to come up with this color for the buying public. But ... when naming a new plant, putting the word “blue” in the title will certainly help sell the product.
There are several perennial blue salvias.
'Rhapsody in Blue' Salvia
'Blue Hill' Salvia
And my customers love the “Black and Blue salvia, which is an invasive perennial in places like Texas, but in Vermont it is strictly an annual.
'Black and Blue' Salvia
The blue tradescantia does not spread and take over like other spiderworts.
'Zwanenburg Blue' Tradescantia
'Blue Ice' Amsonia is taller than the species ( amsonia tabernaemontana) and much showier, but I like both of these plants.
'Blue Ice' Amsonia
‘Summer Skies’ siberian iris is a nice light blue.
'Summer Skies' Siberian Iris
The popular ‘Blue Wave’ petunia is definitely purple from my perspective.
But, when combined with other colors, you can get away with calling it 'blue.'
I plant various “blue” flowers as “cuts” because the darker color compliments just about every other color in a bouquet. Among my favorites are 'Victoria' blue salvia and and any of the tall cutting ageratums.