> To pretend this is about slowing down bots is disingenuous as best.
I'm not sure you have a good understanding of what happens to internet services when they don't throttle spam. They become completely unusable.
If you have access to the server's information, it gets even better. Origin makes it much easier to identify likely spam, previous interactions with the site and their speed ("hits the page and 1s later submits a comment") provide more info.
Sure, all of that can be worked around, but that makes it more complicated and increases the cost for the attacker. If they spend money on faking actual user interaction with your blog, routing all requests through a residential IP in your country etc pp, they are likely spending as much or more than they would on a recaptcha solving service.
If clearly spam, block it, if clearly okay, allow it. If unsure, leave it for a moderator. It might even train users to write better comments if badly written ones need to wait for moderation.
Also, anyone who mentions <something the service wants to censor> can be, of course, blackholed as spam. ;)
The submission itself should be done by the site, so the user remains anonymous.
Any spam detection engine that asks for that information will outperform ones that don't.