It's my understanding that when visiting a site using TLS/SSL, all information contained in the URL is encrypted. That is, visiting https://reddit.com/r/darknetmarkets will handshake with the server hosting reddit and the rest of the URL (subreddit information, etc.) is only visible to reddit's servers after the handshake is complete. Am I mistaken?
Yes, the entire get and post requests are encrypted. However, reddit isn't HTTPS by default so you have to manually type https://reddit.com, get HTTP everywhere plugin, or have an account and enable HTTPS. Unless your a vendor or a larger dealer you shouldn't have to worry about what the ISP sees.