One-click-hugo INSIDE a pre-existing website

I have a website on netlify and the client asked for a blog inside this pre-existing website.
I’m trying to add one-click-hugo inside of it.
The thing is: I can do it, but when I add a new post on netlify CMS, it replaces the index website for the one-click-Hugo index… I changed the index.html (hugo file) to index_blog.html but when I add the new post it just replaces and makes it index.html again.

I’m new on it… can someone help?

Ask Netlify support. This is specific to their product.