So there were a few things I found interesting this chapter. Particularly this sequence:
I noticed that he was looking at his right hand and the panel seems to be drawn from the perspective of his right eye, his closed eye. The following panel shows his face obscured in shadow but his left eye is open.
This might be me reading too much into it but I went back a few chapters and it was mentioned that this chamber is in absolute darkness and silence.
In fact we even see him stumble and fall into the pool and we are given the above panel symbolizing his physical and emotional isolation. In retrospect, even entering the chamber is a clear analogy for death - it's a tomb and the characters call it as such.
But what I find interesting is that after the realization that he's half-astral he is able to see his hand and the "aura" around it - for lack of a better word. Is it possible that the parts of him that are "dead" are now able to interact and perceive the astral world? What then about his missing hand?
Once I started going with that theory this panel seemed more than accidental to me. Notice how the light is coming precisely from where his missing arm should be. Coincidence? I think not!
No, but really, it might just be a coincidence since the framing looks nice. But I'd be interested in how this crackpot theory holds up. At the very least I'm comfortable saying that something about the "power up" made Guts able to see beyond the physical world hence why he's able to see in absolute darkness. It would also be a perfect solution for his eventual fight against Griffith who has a physical body but is a creature who exists beyond the physical realm.