If I read correctly, you want to have an Index page that just says "Hello, welcome to our domain. Go HERE for sales, HERE for news, HERE for the blog", yes? And one of those links is to
domain/blog and your WordPress?
Two options. One is to set your first page to be a Page, while your blog hides "behind" it. To do this, go to Options / Reading and change the first option so that your Front Page Displays: (a static page). You can create this page within WordPress - it's like a Post, but slightly different. (I hope that made sense).
The other option is to install WordPress in a different location (the /blog folder) and have a different, static web page in the root folder. You can move your WordPress install, or just uninstall it and reinstall it there. You can look this up in the Codex - it's linked from the Installation instructions.