Not a dumb question at all :)
To have the website have a global navigation, where all pages in the entire website uses only 1 navigation and not a copy of one you need to have
a nested master page.
In the parent master page just put the contents that would be in all of your pages main site and BE blog pages. Like your navigation and footer.
Then for your 1st child master page have the content of your main site in one child.
Then for 2nd child page master page have the BE content there.
In total you will have 3 master pages.
That solves your navigation part.
For the login issue have a single login. Meaning you have to merge your membership provider with each other so that you have only one.
Then your users only have to login in once to access both the blog and your main site.
Here a useful link to help you out with Single login:
You can also do a search for "Single Login"
Hopefully this gets you where you want to go.
Have a good day,