Lost my iPhone and I'm blocked the iCloud

Blacklisting is done by the cellular carriers. Contact your cellular carrier and let them know you have recovered the device. If it was replaced using some kind of insurance coverage, they won't remove it from the blacklist. It no longer belongs to you in that case, it belongs to the insurance company.

Now.... what does any of that have to do with iCloud?

the IMEI black list is a carrier thing not a phone thing

you need to try to convince them to remove it from the block list