In the MYP series, there were only 4 characters to be blue skinned - Keldor, Sy-Clone, Tuvar and Vormus. Using the multi species world they created, which is now being used prominently, and nothing being mentioned regarding space travel, one can speculate that they all came from the same species of a blue skinned people - Gar.
Keldor being half, Sy-Clone being an android of some sort, Tuvar and Vormus both having elongated ears and red eyes would indicate how they would originally look. We did not see if Vormus had fangs.
Tuvar. I'd classify him as a Gar - a more warlike race of Gar, with red eyes and ridged head but of the same species. Like the Romulans and Vulcans are from the same species yet they are different races.
Kronis was not a Gar for the MYP series but he is in the DC canon and if one is trying to encompass all media into one, like Classics, then one can speculte that he is a Gar as well, just from another region (like a hotter climate) of the planet/dimension of Infinita which would explain his darker complexion. Or you can see him as green skinned and "shirtless"

if you prefer the vintage comics.
Illumina and the Gar council of Elders in the MVC all had the pointy ears but white eyes which could be indicative of a more morally good natured race of Gars, which would explain the non ridged heads or fangs.
Shokoti. a vampiric like sorceress with blue skin. She can be seen as a Gar as well who changed in appearance slightly due to her corruptive nature of using dark magic - black eyes.
The two blue skinned villains shown previously from Filmation - can be associated as Gars too
Sy-Clone - MYP - seem to be an android of some kind left by someone in Anwat Gar to defend the Lagacy stones which he had been doing for what looked like, centuries. Filmation - He wears a suit to fly, a high tech Gar warrior as opposed to the magic weilding ones that are shown as villains.
The ridged helmet that Sy-Clone wears can just be a sign of their ancestry. To add in Strato's Avion people, their helmets can be a homage to the Gar as well. Their technology helped them advance as a species. That was one huge ship they had in that MYP episode where they were going to war with the Andreenids.
To have as many of this (Gar) species to MOTU does add depth to the overall stories. Shows that the Gar were an important species to Eternian history. Maybe they're the ones that stopped the spreading of the techno virus which gave Eternia advances in technology. Who built Central Tower anyways, or is this already known somewhere?
Its always fun to speculate and to bring in certain elements of all media into one canon, which at the end of the day, is YOUR canon. It shouldn't matter what's official or what isn't, especially when the mythos always changes, making it your own is what counts and whats more enjoyable at the end.