Based on your question, I would suggest that you are looking for a common format into which all the big boys are exported, which can then be used by your application. From what I see, WordPress eXtended RSS seems to be pseudo-supported among WordPress, Movable Type and Blogger. You might want to read more information here: http://codex.wordpress.org/Importing_Content
This link does a double duty for your question, as it also contains a complete list of common blog engines, their export assumptions and what formats you should consider.
As for the WXR file format itself, it seems rather difficult to find the specification in Googleโs own database and WordPress. Here is the most suitable result that I could find: http://olalindberg.com/blog/2008/12/13/write-your-own-wxr-file-to-migrate-custom-cms-to-wordpress
Good luck
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