There are people on here who can help you out with how to tell the two subspecies apart...

But, a 'Florida Blue' is a morph of Eastern garter with patterning very similar to a Similis or 'True Florida Blue'. Similis will tend to be more blue, but that's not a great indicator of species. Florida blue easterns can also be greenish or olive colored. The main difference I've noticed is that Similis tend to be very pissy
They are often confused, especially by people who aren't really familiar with garters. Not only are they probably accidentally interbred in captivity with Fl. blue easterns, but they almost certainly interbreed in the wild as well since the similis is a subspecies of Eastern garter and their range overlaps completely.