Does Blocs have a staging site or site that you can publish to like bootstrap studio
No, you’ll need to set up something with your own hosting. I keep a dedicated domain for this on my server with instructions to block search engines.
If anyone than you knows the URL, search engines won’t find it.
Google doesn’t have a index of all available domains. Either you register with Google Search console, or Google follows a link from another website to yours.
It doesn’t take much for a link to appear somewhere if you are sharing it with clients. I simply use a robots header in the htaccess and that seems to block sites from appearing in the indexes.