Hi, I'm Diane and I'm happy to help you today.
The new Outlook that replaces Windows Mail does not currently support blocked list for IMAP account, only outlook.com.
If you need to block mail, you need to use the classic Outlook desktop software that is installed with Office desktop software.
do you have Office software installed? If yes, Type Outlook on the start menu - do you have 2 listed? You want the one that does not have the "new" banner on it.
You can leave Feedback from the Help tab > Feedback or in the Outlook Feedback Portal at https://feedbackportal.microsoft.com/feedback/forum/89a8afa3-2e1c-ec11-b6e7-0022481f8472
Let me know how it goes, I'm here to help you further if needed.
-- Diane