Generally it’s not recommended to embed Discourse within another page (using iFrames for example) — this isn’t supported and can come with unforeseen complications.
It would be better to have the forum on a separate subdomain (like forum.example.com and blog.example.com), and include links on each site to connect them.
To make this more seamless, you can use Discourse as a commenting system that embeds comments on your blog and links to the forum to continue discussing. There’s some more information about that here: Embed Discourse comments on another website via Javascript