Multi-site with two sites sharing the same domain?
I have done this before. Given a generic location / { ... } entry for Nginx which serves domain1.com to make the subdirectory install work I did the following 2 steps. I am not aware of a solution that doesn't involve making a symlink.

Step 1:

In your Nginx configuration:

    location / {
            # First attempt to serve request as file, then
            # as directory, then fall back to displaying a 404.
            index index.php;
            try_files $uri $uri/ @rewrite;
            expires max;
            # Uncomment to enable naxsi on this location
            # include /etc/nginx/naxsi.rules
    }

    location /blabla {
     try_files $uri $uri/ /blabla/index.php?$args;
    }
    location ~ /blabla/.+\.(jpg|jpeg|gif|css|png|js|ico|xml)$ {
            webuser /usr/share/nginx/html;
    }


Step 2:

Then wherever your docroot is for your site you add an alias with your subfolder name to the main docroot, allowing the path to be valid for Drupal:

$ pwd
/usr/share/nginx/html

user@localhost:/usr/share/nginx/html$ find . -type l -ls
41059    0 lrwxrwxrwx   1 webuser    webuser           21 Sep 25 01:58 ./blabla -&gt; /usr/share/nginx/html


You will still need your sites.php configuration as you have listed. You will need to restart Nginx.