From the Library of Congress:
An ISBN or International Standard Book Number is the book counterpart to the ISSN. It is a national and international standard identification number for uniquely identifying books, i.e., publications that are not intended to continue indefinitely. A publication can have both an ISSN and an ISBN. This situation occurs most commonly with books in a series and with annuals or biennials. The ISBN identifies the individual book in a series or a specific year for an annual or biennial. The ISSN identifies the ongoing series, or the ongoing annual or biennial serial. ...
The ISSN can be thought of as the social security number of the serials world. Just as an individual's social security number is used in many automated systems to distinguish that person from others with the same or similar names, the ISSN distinguishes a particular serial from others with which it might be confused. The ISSN also helps library patrons, libraries, and others who handle large numbers of serials to find and identify titles in automated systems more quickly and easily. ...
There is no charge for the assignment of the ISSN, or for the use of an ISSN once assigned. (However, the Library of Congress incurs substantial costs to staff and maintain the U.S. ISSN Center. Additionally, the Library of Congress is assessed a considerable fee to belong to the ISSN Network.) ...
There is no connection between Copyright and ISSN. Having an ISSN does not confer any Copyright protection, nor does sending a serial to the Copyright office eliminate your need to send the U.S. ISSN Center a sample issue of a serial for which you were given a prepublication ISSN. ...
Getting an ISSN for a title does not confer any exclusive rights to that title. Nor can titles be copyrighted. The best way to protect a title is to register it with the U.S. Patent & Trademark Office.To register your blog online with the Library of Congress, go here.
One more thing: thanks to Lynn Viehl at Paperback Writer for the head's up.