Rek'sia has no eyes, yet is only blind underground

As you said, she has no eyes to beginn with so she is not exactly "blind" either.

Afaik she uses some kind of echo location, but underground she can't send out any soundwaves, so she only listens to incomming signals.

logic

(Rengar is half way to Blind Club)

cause #realism

Rek'sai has no eyes, yes, but there are a lot of real life animals that are either naturally blind or in possession of vestigal eyes, and those creatures have adapted to be successful in their niches. Look up golden moles - "their eyes are nonfunctional and covered with skin and fur." Bleh.

When underground, no one can see squat on the surface without cameras or stuff; Rek'sai doesn't have those and doesn't need those. She appears to be sensitive to ground vibrations and must be hunting via echolocation, like bats or dolphins.