A "dark webhook" in this case just means sending data from a public server/service like a GitHub action (or even an AWS Lambda) to a server that doesn't have a port open on the internet.

For example, I get a message to my Mattermost client everytime someone pushes/commits/etc. All those messages are sent to the Mattermost server over a webhook that is configured on an overlay network (in our case, using OpenZiti). There's no firewall rule allowing the traffic to the Mattermost server, it's "dark" to the internet.

I assumed it would be something about the "dark web", never heard the term used for services that are behind a firewall. TIL.