It's not that it's a paradox, so much as a contradiction by definition. If the darkness is fully transparent, then it can't be seen
at all, by definition. If the darkness is instead partially transparent and partially opaque, then it can be seen (if it's backlit, anyway) because it partially blocks the light from behind that is passing through, making that light dimmer.
Any level of opacity can be made to work, I just am not sure what level of opacity you have in mind, since you seem to be saying the darkness is simultaneously 100% transparent and still visible.
Let's say that darkness is cast in midair (it can't technically be cast in midair, but assuming it could be for sale of argument) on a sunny, cloudless day. There are no objects or creatures in its area. Under your approach, do observers on the ground looking up see:
- An uninterrupted bright blue sky? (darkness is 100% transparent)
- A bright blue sky interrupted by a 30' wide circle of dimmer blue? (darkness is less than 100% transparent, but greater than 0% transparent)
- A bright blue sky interrupted by a 30' wide circle of black? (darkness is 0% transparent)
(For reference, the clearest ordinary glass is about 85% transparent, if I remember correctly.)
From your descriptions I'm pretty sure you run Darkness as #2, partially transparent. Is that correct?