This is not a burnt hand, this is a hand or even body that took to much blood and now the veins and arteries are visible.
I mentioned the top of the arm. it's black like burnt. When Sirius called Violet "Come here child. See
http://www.mangago.zone/read-manga/kuroshitsuji/an/v23/c115/pg-23/
That must be the shadow
:)))
If it were Vince than Undertaker wouldn't still be morning his loss, he would know if he was alive. That hand to me looks like that of a old guy, or weakend Vampire.
I don't know Why but I think master Sirius is Vincent Phantomhive. Because that hand was burnt... (⊙…⊙ )
Excuse me for my bad English.