How can I blacklist my stolen iPhone IMEI number?

This is a user-to-user forum. Neither we nor Apple can blacklist your iPhone.

If you had Find My enabled on your iPhone when it was stolen, you can mark it as Lost and request a remote Erase.

If your iPhone or iPad was stolen - Apple Support

Do's and Don'ts for lost iPhones - Switched Off - Offline - Beware of Phishing – Apple Community

As far as blacklisting your phone, contact your phone company (carrier). They are the ones who do that, and they might want you to file a police report first.

The carriers black list IMEI numbers, not the manufacturers. Please contact your carrier.

Only your carrier can blacklist the phone. Contact their support folks.