|First appearance||12: Poisson|
In his talk at Dartmouth  Randall discusses his reasoning behind creating a separate "asshole" character in his comics, or a classy asshole, which his friend Beth coined Classhole, as mentioned in the title text of the comic of that name, where Black Hat describes how he aspires to become just such a Classhole.
Unlike many other characters in xkcd, he seems to represent the same character in every appearance.
It is possible that Black Hat has a name that has been used in the actual webcomic, as opposed to the name "Black Hat." In the title texts of 29: Hitler and 493: Actuarial, Black Hat is referred to as "the hat guy" and "Hat guy" respectively. In 1139: Rubber and Glue a young Hairy also gave him the nickname "Hatboy". These early characters may also be a different persona.
It's not verified that Black Hat is a reference to Black hat hacking, but the character's personality is in line with the idea of a "black hat hacker" from the software engineering culture that Randall's comics are written for. Black Hat speaks from a place of casual power and safety, living in the luxury that many early hobby hackers found by being ahead of the tech curve or even actually hacking financial systems, hatching evil schemes often for no reason other than to cause chaos or go against the societal norms.
As stated in the title text of 29: Hitler, "the hat guy" is based on a character named Aram from the now-defunct webcomic Men in Hats. Like Black Hat, Aram seems to have frequently made judgmental, insulting or controversial comments in a very emotionless manner. This may also have been the comic where Black Hat truly comes into existence for the first time.
In 24: Godel, Escher, Kurt Halsey, the first release chronologically using Black Hat (see the trivia) (a comic drawn during an 'unending Nasa lecture'), Black Hat does not really resemble his later appearances.
Judging by 1139: Rubber and Glue and 1753: Thumb War, he has worn the hat since he was a child. Within the latter strip, Black Hat seems to have short hair, as also shown in the "Journal" series, 412: Startled and 1401: New.
While no definite reason is given for how he became so sadistic, in 792: Password Reuse Black Hat says "since March of 1997 I don't really believe in anything" suggesting some traumatic experience made him this way. Some black hat hackers are simply so detached from the real world they don't understand that others suffer; others may have led rough lives and been through crime rings as a sole way to find community. In 1139: Rubber and Glue, the boys making fun of Black Hat as a child appear to be in elementary school based on one calling for his mom, so Black Hat probably is too. He is already sadistic by this point so the event would have happened when he was quite young; experiencing it so early would explain how the mystery event could have reshaped his entire personality. Unfortunately no details are ever given.
- Actually the first comics to be released using Black Hat is not number 12, 12: Poisson but 24: Godel, Escher, Kurt Halsey because the latter was released on LiveJournal (as number 6), where Poisson was first released when going over to xkcd. See details in the trivia for both Poisson and Godel, Escher, Kurt Halsey.