Prevent my Mac from connecting to an unsecure Wi-Fi network?

Your Mac will only automatically join Wi-Fi networks which are in its preferred networks list.

If the non-secure network is not in that list, it will not join it.

If you want to be prompted to join a network when no "known" network is available, turn on "Ask to join new networks."

However, if you do not turn on "Ask to join new networks" then your Mac will not automatically join the non-secure networks, it will simply stay disconnected without prompting you.

So there is no need to 'blacklist' a network because it will not join it unless you tell it to join it. Unfortunately there is also no way to tell your MacBook not to present it as an option. The best you can do is require an administrator password before it can be joined: