Steven Moffat and Mark Gatniss like to create elaborate universes for their fanbase. This includes having the BBC create websites that exist in the worlds of both Sherlock and the fans. The following list only has official blogs for Sherlock characters. There are no weird fetish Tumblrs or LiveJournal entries about what someone would like to do with Sherlock, John, and a tub of Cool Whip.
The Science of Deduction
Sherlock’s website The Science of Deduction is identical to the one he updates in the show. It comes complete with a request for “interesting cases only please” and case files. Many of the case files direct those interested to John Watson’s blog because, according to Sherlock, people are more interested in what John has to say. Unfortunately, there is no phone number for female fans to call Benedict Cumberbatch asking for him to deduce some things.
The blog of Dr. John H. Watson
Sorry, Sherlock, but you’re right. John’s blog is more compelling. Prior to meeting Sherlock, John had nothing to write about, except for the occasional meeting with his mates. That all changed on January 29 (no year given). Since that fateful meeting, John has been blogging about his adventures with Sherlock. The posts correspond to specific Sherlock episodes, so John hasn’t update his blog in a while, but that should change when the first episode of series 3 airs on October 31 in the UK.