Deploying two different repos to the same custom domain

I have a repo for a blog that’s separate from my existing site but i would like it to be accessible as a subdirectory of my current site i.e. at I added a 200 redirect rule to my existing site and for the blog site moved all the files and directories into a new ‘blog’ directory; however the url still isn’t working.
I’m using Netlify to deploy (not Gitlab Pages) and using their proxying instructions here:
My question is more if the structure of moving files to the blog directory was correct or if I need to do things differently, or if i’m doing something else wrong that i need to fix?