We don't know his real name. No one does. Yana is intentionally not revealing it because it's not important.
Real Ciel (the new one with Undertaker) is pinning this "Lord Sirius" title on him, because "Lord Sirius" is a criminal identity found guilty of blood trafficking and is wanted by the Yard. Neither of their names are actually Sirius; it's just the name of the constellation the blood type is named after.
We call them OCiel ("OurCiel", the one we all know) and RCiel (Real Ciel, the new one) to differentiate.
Actually, remember when OCiel ate like a pig and lost control? There he gave label.
Real Ciel = Ciel
Our Ciel = EARL CIEL.
I've actually been calling them Ciel and Earl Ciel but when I talk to you guys in the fandom you always use real ciel and our ciel so i conform so that it wouldnt be confusing.
But you can try spreading the Ciel-Earl Ciel labels to spread the word if you want hahaha
That's true. He does have the title "Earl Ciel Phantomhive". But at the same time, RCiel is vying for that title, plus he has the resources to be an earl right now. OCiel is wanted, thus cannot perform his earl duties even though the title belongs to him. So people can get confused about who is actually the "earl" right now.
I think it's just faster to write "OCiel" and "RCiel".

So, who's the Ciel we all knew? I mean his real name, now that we know Ciel is not actually Ciel
Damn, I'm losing braincells reading this