DARK BLUE SNAKE WEED aka Stachytarpheta cayennensis, ark blue snakeweed, blue porter weed, blue rat's tail, blue snake-weed , bluetop, rattail, Cayenne snakeweed, false verbena, nettleleaf velvetberry, nettleleaf vervain
there's also quite a few species of plant that shares the same name
There are different species under the genus Stachytarpheta that look more or less the same. These include S.cayennensis (Dark Blue Porterweed), S.indica, S.jamaicensis (Light Blue). The flowers of S.cayennensis are darker than the other two.