Can it still be a dark pattern if the player understands and accepts the negative experience?

Yes. The problem is not the player's acceptance but the manipulation of their decision-making and the exploitation of psychological weaknesses. A 24-hour "rare item" offer is still a dark pattern even if the player understands it is time-limited, because it is designed to create irrational buying pressure through artificial scarcity.

The negative experience can also happen after the game, e.g. regret about money spent.

Dark patterns often exploit unfair cognitive biases, which work even when the rules are understood. The standard example is FOMO (fear of missing out): a shop offers a rare cosmetic item for only 24 hours. The player knows it is a scarce offer and accepts the time limit. But the design's intention is to create irrational purchase pressure through artificial scarcity, and accepting the condition does not undo that intention.

The open question this leaves: how are players' consent and knowledge established, and are there cases where a trick or concealment is legitimate, e.g. as an extra challenge or for an emotional experience?

Go deeper:

• Wikipedia: Fear of missing out — the bias time-limited offers exploit.
• Wikipedia: Scarcity (social psychology) — why scarce things feel more valuable.