Tbh I think that lord Sirius isn't actually the blue star since Sirius (the actual star) isn't even a blue star. There is, however, a very large blue star directly below it. Giving all the stars the title "lord" would imply that they're all of the same status. It also means that there is a caste system at sphere music hall. But the blue star is never referred to as lord, only the "blue (or pale) star". It's like when there's lords and a ruler, the ruler isn't a lord since he's higher than that.
Sigh… Sirius is a BINARY star system – a huge blue Sirius A, and a white dwarf Sirius B. Sirius B used to be blue and more massive (and brighter) than Sirius A, but then it collapsed, becoming the white dwarf we see today. This is also probably why the Sirius system is being used to denote blood type AB, because it has BOTH.
At this point, it’s hard to tell who is Sirius A… and who is Sirius B. Perhaps Blavat calls Lord Sirius “Blue Star” only when talking to certain people, OR perhaps he’s saying this was all a trap to get the earl, making the earl the “Blue Star”, the one who (now) shines brighter.
Either way, there is the company Blue Star Line, and it is probably controlled by Lord Sirius. Perhaps Lord Sirius considers himself to be Sirius A, but others know he’s now the white dwarf Sirius B.
PERSONALLY, I’ve come to the headcanon conclusion that Lord Sirius is the once-bright blue star who became the white dwarf shell of his former self (when he died and was turned into a Bizarre Doll), while the earl is the remaining blue star who still shines bright…. They revolve around each other, tugging at each other, getting ever closer until the collide, just like the real Sirius A and Sirius B....
About the lords of the stars, they are not given the same status. Lord Polaris is a butler and stays in a destroyed room with few appointments. Lord Canopus is also in a basic room. The two Lord Vegas are higher class in a nice bedroom, and Lord Sirius is in a stately bedroom.... Even Blavat says some stars are a higher magnitude that others, Sirius being first magnitude, Canopus being second magnitude, etc. Ch127 suggests that Lord Polaris is actually the butler to Lord Sirius. They are only called "lords" because they are the representatives of their stars/blood types for the purposes of the Blue Sect....