Please Read: How Bloodspot works

[Webmonkey]

Per Anarchs Unbound, pg. 82

Bloodspot
In 2004, programmers allied with the Red Question (a Kindred hacktivist group) introduced a new program called Bloodspot. A significant improvement on ShreckNet 2.0, Bloodspot alters the HTML code of a particular web page to allow it to intuitively identify whether the user is a vampire or not.

To non-vampires looking at the screen, the page displays as “file not found.” For Kindred, the true page is plainly visible without any Auspex. Although eagerly adopted by Kindred around the world, Bloodspot is not without...

And later on down the page:

Regardless of the risks, Bloodspot has been widely accepted by technophile vampires across the world, and tonight innumerable Kindred bloggers, some of whom (like the anonymous author of the satirical Book of Donhumor blog) have cultivated thousands of followers across every sect and clan line.

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• Non-vampires (even ghouls) cannot see Bloodspot pages, but any Kindred from any Sect (with the web address) can.
  
• Bloodspot blogs are pervasive enough that any Kindred with a Computers rating can have one without fear of breaching the Masquerade. It's 'automatic' in detecting if a user is Kindred or not.
• The Bloodspot app was built by Anarch technomancers on top of Nosferatu Schrecknet 2.0, which is open to all Kindred in V20, with all of the political and mystical connotations that might mean.

For more in-character information on how technology like Bloodspot works in V20, please see a vampire storyteller to make the appropriate Intelligence + Computers rolls. For OOC stuff, look at the Technology sections of the V20 Companion and the Anarchs Unbound books.