If you're planning send out mass emails, you'll probably want to look into using a 3rd party email marketing tool such as Vertical Response.
They provide you with the tools you need to not only create a professional email, but also a subscribe widget. You won't be able to add html to a blog post, so your viewers cannot subscribe directly from a blog post. However, you can create an a subscribe widget and add that to the same page that you have your blog on.
Outside of using a subscribe widget, you can create a custom form and have the information emailed to you. You can then manually add their email to your mail list.
To build a custom form in Sitebuilder Plus, click +insert > more > build a custom form
Hope this info helps, let me know if you have any other questions.