How to search or filter unknown blocked contacts in Apple Mail on Mac?

" Am I missing something? "

No

I have just tested your steps, also using Tahoe 26.2 and get the same results

This is practical way to Export or Copy and Paste all the Blocked ( Unknown ) Contacts in macOS 26 Tahoe

Do you have Phone Number of this company listed in you Contacts Application ?

It's a pointless and never-ending task trying to block spammers locally in Apple Mail because they just change their email addresses when the commercial spam blocking services start to block them. Your best bet is to use whatever junk-mail filtering tools your email host has; if your ISP provides such tools they will normally found in the settings page when you sign up on the webmail interface in a browser. Then you can use Apple Mail's junk settings to put it items marked junk into your junk mailbox so they don't clog your inbox and empty the junk every few days.

Good work but Op's issue was " But there are so many unknown blocked contacts that I can't easily tell if this company is on the list "