Can I use Brightwater to return Darkstar to it's owners hand?

" after a card is discarded from a players hand, kneel BK to return that card to its owners hand "

I wasn't sure how the timing would work - Darkstar is a passive

If they were both responses, would the first response be successful and the second unsuccessful

I also assume I cannot use BK to return a card discarded from hand as a cost ~ because there is no window to do this?

thanks

Yes, you could use BK to return Darkstar to hand as Darkstar is still counted as "discarded" and it is still during the same Action Window. If both were Responses, you'd still be able to do it because you would still have an opportunity to trigger the response once your opponent has triggered theirs.

Technically, I believe you could still use BK when a card is discarded for a cost because BK is not a cancel, it's a Response. The cost was still successfully paid, so you should still be able to trigger the response for the card being discarded. If it were a cancel, you'd be right, there's no cancel opportunity for paying a cost.

You can use Brightwater to return a card to hand that was discarded to pay a cost. You have to wait until the effect that it paid for has resolved (Step 3) before using the response (Step 5), so it's not like you "undo" the cost and stop the effect, but you can get it back.

Your "no window to do this" thought is a misapplication of the fact that there is nowhere in the timing structure to save a card killed or discarded from play as a cost. Saves must interrupt the initiation and resolution of an effect. Costs don't have separate initiations and resolutions, so there is nothing for the save to interrupt. When you kill something for cost, it dies immediately (Step 1). So by the time you could trigger a save (Step 2), the card is dead/moribund and too late to be saved.

You can see that that has nothing to do with discarding a card from hand to pay a cost, then responding to that discard with a normal response. You're not trying to interrupt anything, and there is no moribund involved. Just be aware that the card leaves your hand and becomes part of the discard pile entirely before Brightwater is triggered.