I haven't grown this one myself yet, but Stacy down at Broadturn Farm brought it to my attention.
It needs the same cultural conditions as the Campanula medium types, so plant it early, (before week 12) when the days are still short. Long days will trigger flowers, and you want some time to establish roots before that happens. The "bells" are small and dainty and come in big clouds of bluish lavender.
I expect them to flower around the time of the other Campanulas, which is often a time when there is often in that early summer gap when the spring flowers are gone.
As with all campanulas, plant on arrival as they have a tap root that can easily become root bound.