That is a very good question! The ISSN Number is an official, standardized designation for a periodical, either in dead tree edition or digital. You can read more than you have every wanted to know about them on Wikepedia.
Around 2002 the issuing body for ISSN's was allowing blogs and online journals to request an identifier, but they have since stopped the practice, although I am not sure why. Possibly due to the transient nature of blogging.
There are various methods by which one can search for data relating to an ISSN number, but most of them are subscription based. One free service that you can play around with is WorldCat.
Some examples of Blogs/Journals you can find via ISSN:
- SIllyness Spelled Wrong Intentionally — Hey, it's a Me!
- Jeffrey Zeldman's The Daily Report
- The Blog of Evan Sims
Wow, Jeffrey's entry is even older than mine. Much older :)