Dear Diary
Dear Diary
[Black Hat is seated at a table, writing with a pencil in a diary.]
Writing: Dear Diary, Writing: Hello. I am the Crown Prince of Nigeria. I have recently come into a large fortune, but...

Black Hat is writing in a diary (probably his, but possibly not; see below).  His entry starts with the common idiom "Dear diary". In a regular diary entry, this opening is used to give the impression of writing to a trusted friend, the diary being anthropomorphized to take that friend's role. However, where other people would write about their day or put their feelings into words, Black Hat's diary entry consists of a standard phishing scam attempting to request some private information in exchange for a large cash amount which does not exist. In this case, the scam is the infamous Nigerian Royalty scam , where the 'royalty' needs bank details to give money, when it will in fact be taken. Black Hat apparently is so used to tricking people that even his own anthropomorphized diary is not safe from his pranks. Alternatively, the entry is intended for anyone who looks at his diary without his permission. It's also possible he has obtained someone else's diary and is somehow trying to scam the diary's owner, although it's not clear how that might work.  Or, since it is a rather obvious scam, he may simply being trying to scare the diary's owner, perhaps the same child as he traumatized in the previous comic with a reindeer mutated to look like a spider.
This comic creates a stark contrast by putting together elements that seem similar, but do not belong together, for comedic effect.
It is possible that the diary is actually the journal from the Journal series, and that ever since being outdone by Danish , he no longer uses it for recording all the things he would say if he were nice.
The title text is similar to 1675: Message in a Bottle , which also uses the word "unsubscribe" in an unusual way. The title text also mimics a standard way to get off some mailing lists, so perhaps it's Randall 's diary that Black Hat is molesting, and therefore the title-text is Randall expressing a desire to be disassociated from it.
[Black Hat is seated at a table, writing with a pencil in a diary.]
Writing: Dear Diary, Writing: Hello. I am the Crown Prince of Nigeria. I have recently come into a large fortune, but...
