Levi's secret as a Blind Daywalker could be hidden from normal citizens, but not Daywalkers or Nightcrawlers... after all, how could he hide it when he was permanently connected to the Shadow dimension, even if he used his Harmonic Spine to paint his world of darkness?
The condition of having a spiritual link to the bridge of darkness was based primarily on vision. Although Levi called his way of viewing the world 'auditory vision', in the end, he was still perceiving the world through sounds and frequencies, not light.
The colors were a construct of the Harmonic Spine delivered straight to his mind, not actually painting the surrounding darkness.
Without light, darkness prevailed...
That's why, as Levi stood in the center of the arena, with frequencies painting his world, he was capable of seeing both humans and nightcrawlers gathered around them... he couldn't see only the nightcrawlers, which were summoned in their weapon form.
"A fool?" Levi chuckled.
