Well the blog has a long history and was only recently split off from the core of mojoportal. My guess is any reference to excerpt column was already removed before we split the feature off from core so you don't see an changes related to it in the blog procs (they are probably somewhere in very old upgrade scripts for mojoportal core). The original Excerpt field was a varchar and the implementation was poor because it could truncate html and lose closing tags. So recently we re-implemented an Excerpt but used a new text field named Abstract and finally removed excerpt to avid confusion.
All schema changes do happen in the upgrade scripts including changes to procs. In many cases the select procs have select * which makes it require less maintenance, so in some cases only insert and update procs have to be updated when adding a new column.
Hope it helps,