Simple
The light in the waters vanished as he stored the blood crystal away. At the same time, he sent out a pulse of Bloodline Resonance.
It all happened in an instant.
The waters were agitated beyond recognition. Their senses seemed to tell them that Ryu was everywhere and yet nowhere at the same time, all while they had lost the light source that they had been relying upon before.
The blood crystals seemed to be the only things that could light up these rotten waters. Even those with light or fire abilities found themselves out of luck in this regard. So the moment Ryu put the blood crystal away, it was like they had once more been thrust into the jaws of hell.
Ryu's pulse of Bloodline Resonance covered two bases for him at once. First, it obscured him from their senses, and second, it allowed him a perfect lay of how they were reacting.