I don't know if it still does it, but mailinator used to have a neat feature where it would "detect" bots trying to scrape its list of domains and start injecting legit ones like hotmail.com/gmail.com
you don't need to scrape anything. you check provided email for mailinator's MX records\IPs.
This is probably much more difficult and taxing. On every signup, you're now checking MX records and IP's, not just looking at a string. And Mailinator can change this information regularly too if they choose.
>the op's insight is that sites won't blacklist gmail
Why would you blacklist gmail.com when you can blacklist 'GuerillaClick@gmail.com'?
well for one you also need to blacklist the aliases, ex. guerillaclick+sdf@gmail.com, and i suspect sysadmins start showing more hesitation when you need to start pattern matching (risk of false positives and additional overhead)