What plant is that?

The mystery plant for October is a perennial which grows up to 2’ tall and has opposite, sessile and entire dark green leaves up to 4” long and 2” wide . Unlike most flowering plants the petals on this plant do not open and the plant is pollinated by insects which are able to force their way beneath the petals. The flowers occur primarily in terminal clusters and the blue to violet colored petals are five in number. This plant grows in meadows and trail edges and blooms from late August to the middle of October.

It’s. . .