From the Pullip website:
Sebastian is the Black butler from the 3rd version "Kuroshitsuji" manga, Book of Circus. In the series, he transforms into a slightly different version, named 'Black' to sneak into the circus in town for an investigation. We have carefully created the look from the original series. Since all three manga series have the same looking Sebastian in normal life, we've decided to go with the 'Black version' for the 3rd series. This type has several different looks compared to the original type that we've released a few years ago:
And just like the original he has red devil's eyes and a mark on his left hand that shows his contract with Ciel.