Short answer is: This is not directly possible.
Long answer: You’re trying to host the same domain on multiple hosts - this is not possible. A domain cannot point to multiple hosts at the same time. So you’d have to either move the entire domain to Netlify, or keep the entire domain on NameCheap. With that being said, there are 2 ways around this problem:
Use a subdomain: Instead of keeping your blog at https://sualsoft.com/blog/, you can point it to https://blog.sualsoft.com/. So the apex domain can continue using NameCheap while the blog subdomain could use Netlify.
Use rewrites: You can “fake” the URL in the address bar. While I don’t know if/how this can be setup on NameCheap, but the way it would work is that the browser will show https://sualsoft.com/blog/page1/ in address bar, but the page would actually be served from https://shellvpn.netlify.app/page1.