He can see in the current darkness! It might seem a little darker than normal, but it shouldn't impede his vision. As for his keyblade - the overall light power of the keyblade itself will be weaker, but it'll still be usable. Basically it's attacks just aren't hitting as hard.

As for Chongyun, he can use fox fire while partially transformed! It'll just be stronger if he's fully transformed. Hope that helps!