BlogML, an open source project hosted on CodePlex, is an XML format tool for storing the entire content of a blog, you can use BlogML as a way for archiving or transferring content from one blog to another, even from one blogging engine to another. Now, BlogML 2.0 available here.
There are currently several blogging engines that provide support for BlogML including:
I'm remembering the time I had been posting blogs on Pebble blogging engine, an open source project in Java hosted on SourceForge, so very strongly recommended blogging engine, you can download your all content on blog in a zip file. While now, BlogML do the generally way.