One of the 'downsides' of the way Blosxom is implemented is that is relies on a post file's modification date to assign a date to the post. This makes editing files a little tricky: editing files on a Unix box updates the modification date and therefore promotes the post to the top of the blog. I don't know if there's a better way to do this, but I've written a little script that edits a file, making sure to restore the previous modification date.
The implementation is pretty simple: it uses mktemp to create a temporary file and touch -r to copy the modification dates from the post file to the temporary file and back. Be sure to modify the #! line to point to your installation of bash, should you decide to use this script. A nice generalization of this script would have it prompt for a description of the update and add the text to the post.
#!/usr/local/bin/bash
if [ $# -ne 1 ]
then
echo
echo "This script expects a command line argument: the
echo "name of the post to edit."
exit 1
fi
DATE_MARKER=`mktemp /tmp/date_marker.XXXXXX` || exit 1
touch -r $1 ${DATE_MARKER}
${EDITOR} $1
touch -r ${DATE_MARKER} $1
rm ${DATE_MARKER}