I challenge that theory. There is no way to get Heatran kalos-born or kalos-caught, yet he has a blue octagon. My uxie has a blue octagon as well. Only time a blue octagon does not appear is when it is a hack. There are dialgas on the GTS (cannot be born/caught in kalos, except through an egg, but that's rare...) with blue octagons.
Actually all those legendaries can be caught in the new Omega Ruby and Alpha Sapphire. Thats why i said newer Gen VI games. By that i meant Pokemon X, Y, Omega Ruby and Alpha Sapphire. I'm currently sifting through all my pokemon now, and for example, I came across a Volcarona in a Level Ball with a blue pentagon.