Looks to be in different places depending on the account type. And changes depending on the version?
For Classic Windows
https://answers.microsoft.com/en-us/outlook_com/forum/all/i-cannot-add-senders-or-domains-to-the-blocked/e54c4c03-34b1-49b9-882e-9d2c691f31b0
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Windows Messaging Subsystem\Profiles
For Classic Mac
https://answers.microsoft.com/en-us/outlook_com/forum/all/outlook-mac-2016-blocked-senders-info-file/f6fea4b3-09bd-4ff8-a150-89ec518e4109
And changes to the local registry/DB may not be saved.
https://answers.microsoft.com/en-us/outlook_com/forum/all/outlook-365-blocked-senders-list/9e10ddae-ceb9-4dec-a23e-4f13f7ea51be
And if they are stored in a hidden message in the Default Store, you will have to dig it out with MFCMAPI?