Hi Tom,
Based on that article, it sounds like you're trying to give leads the option to sign up for blog newsletter email updates (if not, please clarify). All you have to do is create a "Blog Subscriber" checkbox field and place it on your forms. You can add it as a checkbox to ALL forms with a label like "Yes, I'd like to receive your Blog newsletter", or if you have a "Blog subscription" form, add the checkbox as a hidden field set to true, and on submit, that lead will have that checkbox checked.
When you send blog newsletter emails, use the checkbox as a filter on the broadcast (Blog Subscriber = true).
Hopefully that answers your question, but if not, please elaborate and I'll be glad to help!