Just to be certain: If the first mode is chosen, then no one discards any cards, even if black mana was paid?

That's a weird wording. The first clause seems completely subsumed into the second one, unless there's ever some reason to damage someone but not force them to discard. A madness set maybe?

But even then, a 'may' is probably simpler.

I think V missed that the first mode hits only creatures and the second mode hits only players.

I can definitely see the appeal in a split card of Blightning / Lightning Strike. It's a little fiddly to word, though.

Super mechanical and very inelegant if not just plain ugly design. I have a hard time seeing an environment in which this would see print. Time Spiral 2 block?

Maybe just go with something like (wording tips from Chain of Plasma):

> Sorcery?
> ~ deals 3 damage to target creature or player, then you may pay . If you do, that player or that creature's controller discards a card.

or

> Instant
> ~ deals 3 damage to target creature or player. If was spent to cast ~, that player or that creature's controller discards a card.

Kicker obviously also works.

Maybe "~ deals 3 damage to target creature or player. If it dealt damage to a player and was spent to cast it, that player also discards a card"?

But still a little fiddly.