Since darkness is not energy, but rather it is a lack of light energy, you would need to have your magical darkness absorb the incoming light and convert it into some other form of energy that your plants could use for photosynthesis. This energy type would need to be invisible to the human eye to get the darkness effect you want.

The biology of the leaf would change based upon what you chose the energy source to be. Plants are green because the chlorophyll absorbs the red and blue portions of sunlight while reflecting the green. Your plant's pigment would change accordingly. If they didn't require visible light at all they might turn white since they would reflect anything that wasn't your new energy source, or they could darken, absorbing all energy to compensate for their lack of energy production from sunlight.

You also need to think about the temperature issue. Without the sun's light, the planet would enter an ice age similar to a nuclear or impact winter. This would also change your plant's biology, as they would need to be heartier to survive in the frigid cold. Of course, maybe your converted energy source allows the heat from the sun to penetrate, just not the visible light. You could also solve this by having your energy source generate its own heat instead of letting the sun's pass through, which might allow creatures with specialized eyesight the ability to use it to see.

Hope it helped, Hawke

Since darkness is not energy, but rather it is a lack of light energy, you would need to have your magical darkness absorb the incoming light and convert it into some other form of energy that your plants could use for photosynthesis. This energy type would need to be invisible to the human eye to get the darkness effect you want.

The biology of the leaf would change based upon what you chose the energy source to be. Plants are green because the chlorophyll absorbs the red and blue portions of sunlight while reflecting the green. Your plant's pigment would change accordingly. If they didn't require visible light at all they might turn white since they would reflect anything that wasn't your new energy source, or they could darken, absorbing all energy to compensate for their lack of energy production from sunlight.

You also need to think about the temperature issue. Without the sun's light, the planet would enter an ice age similar to a nuclear or impact winter. This would also change your plant's biology, as they would need to be heartier to survive in the frigid cold. Of course, maybe your converted energy source allows the heat from the sun to penetrate, just not the visible light. You could also solve this by having your energy source generate its own heat instead of letting the sun's pass through, which might allow creatures with specialized eyesight the ability to use it to see.