By default, when you suppress / blacklist an email id or domain using the suppression list available in the panel, all emails to the same email id or domain are suppressed instantly and not processed for delivery further to our email delivery server.
All these emails would be shown as "dropped" under the Live feed section. Usually, all changes which you do within the suppression list are replicated on a real-time basis within the application however at times you might observe a delay of around 1-2 mins max.

In case, if you are doing changes to the suppression list of your Netcore email API account using suppression API, it takes around 3 hrs to sync the same data in  production environment. As a result of which you might observe some delay in syncing the data in production environment.