It’s not much different from other reverse proxy setups, but I have not configured a reverse proxy setup for Webflow yet. You’ll find some posts here- I understand there is a bit of awkwardness getting the settings right to work with Webflow hosting, particularly with Cloudflare.
It’s important for you to understand what you’re building here.
You’d be running, and hosting two sites. Two hosting fees. Two different template designs. You’d migrate your blog content ( or your main site content ) to the second site, since it will use the new template you want. Then you’d setup reverse proxy so that the two sites behave as one.
/* → Site A Main
/blog/* → Site B Blog
There’s a lot of work there, not just building the new site, but the reverse proxy setup, and making a cohesive navigation.
Here are a few posts & articles to get you started.