but this site allready has Wordpress on a subdirectory that is thriving and ranking, widgets.com/blog
It so happens I'm working on a site today with this exact same structure. :-) Your blog posts and URL's wouldn't change at all. You just create a
page with the URL /blog that duplicates your blog index.
You'd also have to change your "home page" in the WP settings.
You can do a no-risk test of this right now - create a test
page in WP and browse to it. It will be the basis of the rest of your work, and can change out pages one by one. If you're using .html extensions on your current pages, just redirect them to the new one in .htaccess:
ReWriteRule ^test\.html$ /test [R=301,L]
The 301 will appropriately redirect SE's with no loss in any ranking you may have. It just doesn't make a lot of sense to have two databases to maintain.
As for that .htaccess code, which is used by almost all CMS's everywhere, after having been slapped about many times by a awesome member here :-) , it's horribly inefficient. It searches the entire file system twice, which gets to be very intensive when your site gets large. I've actually seen drastic improvements just by changing it to something like this:
# BEGIN WordPress
#intensively inefficient
#RewriteCond %{REQUEST_FILENAME} !-f
#RewriteCond %{REQUEST_FILENAME} !-d
#RewriteRule ^(.*)$ /index.php [L]
# END WordPress
RewriteCond %{REQUEST_URI} !^\/*wp\-admin\/*$
RewriteRule ^([^.]+)$ /index.php [L,QSA]
In your case, you'd probably need
RewriteRule ^([^.]+)$
/blog/index.php [L,QSA]
The # are comments, and I recommend leaving them in for anyone stumbling on it and wanting to set it back.
What's the difference? It says "if the request is NOT wp-admin and it does NOT have a dot somewhere, redirect to index.php." You'll never have files without a dot for the extension. This allows normal acces to all images, etc. , it works perfectly . . . .however if you have any subdomains in the same directory (though don't know why you would) it may require a different approach.
It also may need some tweaking if you have any openly browseable directories like /images/. Though you probably shouldn't.
Give it a try, the result may appear the same but it's better.