[jameshart]

How many little web servers work without issue when their root page is loaded from a path other than /?

[qurren]

If that's your concern you can also do this

    server {
        listen 80;
        server_name "tensorboard.localhost";
        location / {
            proxy_pass http://127.0.0.1:6006;
        }
    }

    server {
        listen 80;
        server_name "blog.localhost";
        location / {
            proxy_pass http://127.0.0.1:3000;
        }
    }

HTTP 1.1 and later will have the browser supply the domain name that was used to access the site, and even though *.localhost all resolve to 127.0.0.1, nginx will pluck out the correct configuration and proxy_pass the correct one.