if the url is http://blog.website.com
then the Robots.txt should be accessable threw http://blog.website.com/robots.txt
I would suggest these steps
- Verify your blog the Google webmaster tools
- generate a robots .txt file with Google webmaster tools
- Upload it to the Subdomain.
There is another way if you are using Wordpress.
There is a All in One SEO plugin / Wordpress SEO by Yoast. threw the settings you can specify to add NOINDEX to all Category, tags, author and others. its faster and error free.