Blacklisting a phone

You would contact your carrier for that. Apple doesn't 'blacklist' iPhones.

Apple has activation lock, which means if you were signed into your Apple ID, the device should not be usable by someone else

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Apple does not get involved with stolen devices.

There is no way to report a stolen device to Apple.

Here is Apple's documentation on lost or stolen devices:

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

6 Contact your wireless carrier

If the missing device is an iPhone or an iPad with cellular, report your missing device to your wireless carrier. Ask the carrier to disable your account to prevent calls, texts, and data use. And if your device is covered under your wireless carrier plan, file a claim.

Beware of a common phishing scam: You may get a text or email from “Apple” or “Apple Security” (or sometimes “Police”) saying your phone has been found and to click on a link to see its location. This is NEVER from Apple or from law enforcement; it is from the thief or fence to steal your Apple ID and identity. Delete any such message without responding to it. Apple never gets involved in lost or stolen devices→Avoid phishing emails, fake ‘virus‘ alerts, phony support calls, and other scams

FYI: Apple’s Activation Lock will prevent the device from ever being used for ANY function … while a carrier blacklist would only inhibit it’s telephony functions.

HOWEVER … you MUST keep the phone listed on your AppleID for this “lock” to be maintained.

If you were to remove the device from your account, the phone will become “unlocked” and available for re-use.

Again: Apple can not and does not blacklist phones.

If your carrier won’t do it, which they are not legally obligated to do by the way, you don’t.