I remember watching a video called "Journey's End: Saga of Star Trek The Next Generation" showcasing the end of TNG where Jonathan Frakes talks about Skin of Evil.
Frakes: I'll never forget an episode from the first season, Skin of Evil. Riker was SUCKED into a pit of black slime! So I asked the powers at be "What is this stuff?". They said "Don't worry. It's fine. It's Metamucil and black printer's ink. Naturally, I dove in. In the pit was an elevator which forced me up through the scum and than vomited me onto the beach covered with Metamucil and black printer's ink. When the scene was over, LeVar Burton as Geordi walked over and leaned down, looked at me and said "Frakes, I never would have done that.".