You can manage to allow or block communication of your Teams users with other Teams or Skype for Business (online or on-premise) domains by accessing
Teams Admin Center > Org-wide settings > External access
Although it is clearly written on the top of the page, some might not notice it.
As it says, all domains are allowed to be communicated with by default.
Simply, when you add a domain (as allowed) to the list, all the other domains will be blocked and any domain you need to communicate with you have to add to the list as allowed.
And when you add a domain to the list as blocked, it means only this domain is blocked and all other domains will be allowed.
This would simplify the configuration and there is less need to have a mix of allowed and blocked domains although it is possible to have such a mixed list.
