Speaking of a “black hole” email address, are there any addresses set up for the purpose of catching spam? Like if Gmail had an address for spam that contributed to its spam detection.
The Stalwart mail server allows for that. They call them "spam traps".
Basically, it's a real email address that literally never gets used or referenced anywhere, thus assuring any email received is unsolicited by definition. Stalwart's spam engine uses any such email to help train the spam filter.
I can't imagine that Stalwart is only one implementing such a system.
I've never used Stalwart, but it's the email server I've selected should I decide to do what everyone tells me I shouldn't: run my own server for me, my wife, and the two domains we control. Their documentation is basically a master class in email.
“Spam trap” and “spam honeypot” are exactly the keywords to search for. I found a bunch of info about some services you can use to set them up. I’d recommend adding “-avoid” to your search filters because every email marketer has their own article titled “About Spam Traps and How to Avoid Them” which just pollutes search results if you’re actually looking to set up your own.