WHO can put my IMEI number on the "blacklist" ?

Since 2012, acting under pressure to do so or be legislated to do so, the entire USA cellular service provider community of companies agreed to implement a universal USA-wide GSM and LTE blacklist. Redpocket Mobile is the one to blacklist their customers reported stolen or lost devices. The initiative was managed by the CTIA - the USA wireless carrier trade association.

USA carriers are required to use the blacklist (well, they “volunteered” to do so when threatened by congress that if they didn’t, they’d be legislated to do so). Call Redpocket back and tell them if they will not blacklist your device, you will complain to both the CTIA (https://www.ctia.org/consumer-resources/preventing-device-theft) and the FCC.

Your only help can come from Redpocket as they are who you pay your cell phone bill to.

Did you have Find my iPhone enabled on your device? If you did, it's activation locked, so no one will be able to bypass that or ever use the phone.

Read this link: If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

If however, you did not enable Find my iPhone and you didn't have a passcode on the iPhone, your phone is primed for someone to wipe and use as their own.