This wget bash command in Linux will go through your archives widget (you need to have one) and download all your blog posts and image.
wget -r --exclude-directories=page,category,author,comments,feed,tag,feed --mirror --page-requisites --convert-links --span-hosts --domains example.files.wordpress.com,example.wordpress.com http://example.wordpress.com
Just replace all instances of example with the name of your WordPress.com blog. The above command does not download comments, feeds, etc. If you would like to have them downloaded too, then make appropriate modifications to the –exclude-directories parameter. If you remove tags and categories, you might get a lot of repeats. So, don’t.