That's just how her eyes look in the source material (she's also a barely-domesticated devil that constantly lusts for blood so the resemblance might be intentional).

It reminds me of a plotline from Toqger where the red ranger slowly started emitting darkness from within himself and got a stronger pitch-black suit until his friends snapped him out of it and helped him get an even stronger RAINBOW suit, I wonder if something similar is gonna happen here?