You can see the interior of the original Blue Grotto in Curtis Harrington's film, "Night Tide", starring Dennis Hopper. The entire movie is on youtube:
https://www.youtube.com/watch?v=sLyoXFHYmpc
It's a very atmospheric movie.
If you go to IMDb.com and type Night Tide into the search box you will see it come up. Click on the movie title and go to the movie's webpage. Scroll down to the Message Board link. Make sure you check IMDb out quickly as they are going to permanently delete ALL of the message boards with a treasure trove of information. The Message Board will no longer be a feature of IMDb.   
There is a lot of great info on this movie on its Message Board.
Another of the stars of the film is the occultist, Marjorie Cameron, and also Luana Anders (Easy Rider) and Linda Lawson.