Editing Blog Index Page

Hi, I’m trying to add an email opt-in form to the top of my blog index so it appears at the top of the page before any of the recent blog posts. I use the MailChimp for WordPress plugin to design my forms and embed them using shortcodes. I assume I can just paste this shortcode somewhere, but I can’t find the right file to edit to do this. Does anyone know which file I would need to edit or how to do this? See the Buffer blog for a good example of what I’m trying to achieve. Thanks in advance!