1-The secret is very simple if you know what to do. You must stand in the bridge before the second level of the Crystal Cave at exactly the same spot shown in the image, at exactly the same camera angle.2-Then you must press E at least 18 times and the message seen on screen will show.
3-Now that you have seen the secret number and message, you must progress the game as normal, then once you reach the Halls of the Masters, go to the Blood Mage's wing, and go to the room shown in the map of the following image. In there, there's a painting on the back wall which, normally, says this: However, if you follow the previous steps, this happens and you get about 600 points added to blood magic!

It's amazing how there's still secrets buried in a 13 year old game, I can't wait to see what else could be hidden in other games.