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Ha, actually I'm not sure if anyone really knows ... this toy has a fairly complicated naming history.
When it was released in '82 in Japan by POPY (without a show, only as a transforming toy designed by PLEX) he was called MR-03 Jet Robo from the Machine Robo series and same for the Best 5 release featuring the yellow ver (note side sticker in jet mode).
Then it was released by Bandai America in '83 as Aeroman part of the Machine Men series.
Then it got picked up by Hanna Barbera in '84 for GoBots (made by Tonka) and named Fitor.
Then it got picked up by Ashi Productions in '86 and developed for the TV series, Revenge of Chronos, which is where he was ultimately given the name Blue Jet and finally got a real standout role vs just being a numbered transforming toy or just one of many good guy Gobots.
Apologies for nerding out but this thing has a special place in my book since the red ver. was actually my very first transforming robot toy ... I remember as a kid just standing in a dazed stupor staring at it in a department store window in Ginza and my mom's friend offered to buy it for me (Â¥600) and needless to say I was beyond elated (I remember her name, Chieko, but I don't remember any of my mom's other friends' names ... haha, go figure).
Since the Revenge of Chronos version had the most character development (expert swordsman, of the Jet Clan, wielding the Sky True Sword, expert martial artist, etc.) is why the name probably stuck the most over all the others and maybe the "Blue" in "BlueJet" might just be a reference to the sky in which his Jet Clan comes from on Chronos.
Aaaand for further plot twist there is actually a lesser known blue version called MRV-02 Blizzard (Machine Robo Variation - 02) that exists!