A blacklist is a list of IP addresses or domains that are known sources of spam; often referred to as DNSBLs (Domain Name System Blacklists). The technology was built on top of DNS and most MTAs can be configured to reject or flag messages that have been sent from a blacklisted IP.
When a specific domain or IP is blacklisted internally, there is a possibility that this is propagated to third-party blocklist (blacklist).
And this record takes time to update and can take up to 48 hours. As soon as you have removed the IP (domain) internally, The third blocklist (the record still needs to be updated and this can take some time to update).
Note: A lot of mail servers work this way (e.g, Exchange Online Protection).
I hope you found this blog post helpful. If you have any questions, please let me know in the comment session.