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).