The search for a perennial blue Salvia might end with Salvia nemarosa ‘Caradonna’. While some Salvias for warmer climates enjoy bright blue Salvias, in temperate regions these can only be grown as annuals. Knowing that blue is hard to find in the garden, ‘Caradonna’ might satisfy a craving for long blooming violet blue, from June to September.
‘Caradonna’ is also known for dark purple to black stems, which can make the violet blue flowers seem more blue perhaps, or saturated overall. Also heightening the blue is perhaps its juxtaposition with grey green leaves.
As one of God’s crayons, ‘Caradonna’ is also cherished for its erect habit. Leaves give off a scent when crushed.
Sources:
http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=b679