Deleted knowledge on my main character who did the questline and went to Freharu in Port Epheria. The button to get the knowledge back was not available. womp womp. Tried on another character and tried after swapping servers. This is not a knowledge you can delete and get back immediately. I'll end up doing the story line again on a new char to get the knowledge again. I did confirm it works if you did not get the knowledge initially while doing the missions you can go to Freharu in Port Epheria to get the initial knowledge, you just can't delete and consistently ask to get it back.

KNOWLEDGE ACQUISITION
Black Spirit

Don't be dumb like me, you can get this knowledge after the quest by talking to the black spirit using "W" Chat option. It did not appear for me though, until I had finished the quest [Calpheon] Pressing Forward