At which point you're hitting it on every request and since it'll be a nice, fast, indexed database lookup you might as well just use session tokens instead of a back-assward blacklist.
As I mentioned, blacklists have different characteristics, some of which are desirable. A blacklist of tokens is (for the vast majority of use cases) orders of magnitude smaller than a whitelist, which means they can often be replicated into memory for in-memory lookups on the webserver.
This is a complex distributed systems topic, and there are applications for both.
There are cases where a blacklist totally makes sense, I will agree with you on that.
"I can access this thing" are totally not that for the 99.9% case. Do the simplest thing that will work--and the simplest thing so very rarely involves public-key cryptography!
