Black Water
The Thaumaturgist can pour a special pitch-black oil into a pool of water, thus turning the water solid black. The black water cannot be seen through from the surface, but it has no other effect. The surface of the water will remain black until the effect is dispelled, the water evaporates or the black water is diluted with a large enough amount of normal water. Ancient Thaumaturgists used this ritual to blacken the water of their moats of pools so their underwater guardians could not be seen.
One pint of oil covers a small swimming pool-sized area. The oil will last for a week, but a large amount can be prepared at one time.
