There are too few flowers that have that intense, true blue color, but Ceratostigma plumbaginoides - let's just call it Plumbago - does, and it will bloom for much of the summer, too. This pretty ground cover's foliage turns a beautiful shade of red in autumn, to match the leaves of the trees, and, to be honest, I like its magenta leaves every bit as much as its azure blooms. Plumbago is a long-lived ground cover that grows from 4 to 6 inches high, and while it appreciates sun, it is not tolerant of overly dry soil. Zones 5-9. One more thing: Plumbago is late to emerge every year, so be patient.