Google will still see sites blocked by
robots.txt, and may even list them in search results.
This is especially the case when entire domains/subdomains are blocked. Google will list links to these along with the text A description for this result is not available because of this site's robots.txt – learn more with a link to https://support.google.com/webmasters/answer/156449 .
They tell us that while won't crawl or index the content of pages blocked by
robots.txt, they may still index the URLs if we find links to them elsewhere. They also give this helpful advice:
To entirely prevent a page's contents from being listed in the Google web index even if other sites link to it, use a noindex meta tag or x-robots-tag. As long as Googlebot fetches the page, it will see the noindex meta tag and prevent that page from showing up in the web index. The x-robots-tag HTTP header is particularly useful if you wish to limit indexing of non-HTML files like graphics or other kinds of documents.
So if you really don't want your pages indexed then make sure to use a META tag or HTTP header. I've found
<meta name="robots" content="noindex, nofollow"> particularly helpful for back-end admin areas and control panels when I don't trust
Disallow: /admin to be good enough.