What is the "blackhole" action group used for?

Netreo uses the concept of a “black hole” for routing messages that aren’t supposed to go anywhere, something known as an “email black hole” (thus the name).

It can be thought of as a placeholder for “none,” since in early releases of Netreo it wasn’t possible to configure some types of checks without specifying at least one group.

It is now maintained mostly for compatibility with older configurations and can be safely ignored.