do you mean you added a new custom meta field? as laid out here: http://www.sitefinity.com/help/developer-manual/how-to-gc-based-add-custom-meta-fields.html
, when you add a field to web.config for a module, you have two additional steps: 1) add the input field to the admin control templates (both insert and edit pages) and 2) add the output field to the public control template.
for blogs, the templates are located here:
regarding a new blog post, if I'm not mistaken, these can only be created in the sitefinity administration, so you'd have to have them login. you could make a link to yoursite.com/sitefinity/admin/Modules.aspx?module=Blogs on your blog page to get them started in the right place.
hope this was helpful!