Senka is a special kind of blind. When people can usually see clearly in the light, she can't see at all.
She used to not see anything at all. But, thanks to shadow magic, she's able to see.
She's only able to see shapes based on their shadows in the light, but is able to see clearly in the dark.
This made the assassin life a bit easier for her.
Senka's eyes are nothing but black, just like the shadows imbued in her.