After reading this, I'm on board with Blue Mage. I was actually heavily convinced that Samurai was going to be added, but when I thought about the indications I was using such as the new Antaganist for storm blood being a Samurai, it wasn't really sufficient information. Just naming some additional reasons that haven't been mentioned that support the Blue Mage Theory:
1) If we are in fact only getting 2 new jobs (based on the tweet), wouldn't it be weird that the only job that exclusively comes out of stormblood would be Samurai? Given the fact that Red Mage is a job accessible in ARR areas, that means we'd unlock Samurai from SB and that'd be it. Honestly that would make no sense. I could see if they were adding three jobs and you could get Samurai and Blue Mage both in SB, but Samurai alone would just be odd because it really doesn't fit into Stormblood/Ala Mhigan lore whatsoever. It makes more sense in Othard/Doma.
2) Stormblood currently seems to be an expansion pack which is borrowing from the world and concepts of Ivalice. You can hear it in its music where there are lots of horns and violins giving sort of a (Royal) sound, and even one of the raids are named Return to Ivalice. With that said, when I try to imagine a job that fits more into the lore and world of Ivalice, I think Blue Mage would be that job at not Samurai.
3) Adding to the Raubahn/Immortal Flames/Blue Mage Theory: Raubahn's weapons are also called Tizona which is the Blue Mage mythic (Relic equivalent in ff14) weapon from FFXI. The weapon style is also a scimitar and not a sword which is a Blue Mage style of weapon.
After giving it some thought, Yoshi-P and the developers like to add jobs that "fit" into the lore of the expansion and Blue Mage actually does that really well, but Samurai? That's a stretch. I am still highly convinced we'll be getting Samurai, but I think it would have to come with another job like Blue Mage if it were to be introduced into stormblood.