Buyer's remorse is a defensive mechanism broadly associated with cognitive dissonance. You'd typically expect to find it where a purchaser had misgivings about their purchase for some reason - either they had difficulty choosing between multiple options, or they weren't sure about some other aspect (the trustworthiness of the seller, the necessity for the purchase in the first place).
For once, the Wikipedia article on Buyer's Remorse has a lot of pertinent information and its references make for some interesting reading.
Given that Buyer's Remorse is a stressor triggered by a negative reaction to the purchase, the user's experience would be improved if it could be removed in some way. As such, I don't believe that making efforts to reduce it is necessarily a dark pattern. However, removing it should be based on the root causes of the problem, not simply masking the symptoms.
For instance, it would probably be possible to lie to the customer about their purchase to make it better align with their purchasing goals, or even to attempt to manipulate the customer's perception of their own goals. While this might be effective, this would be a dark pattern as it involves a deliberate and malicious manipulation of the user in order to affect a purchase.
Alternatively, there are methods which are more ethical. For example:
- Presenting less choice to the customer (see also the Paradox of Choice) could, according to Barry Schwartz, reduce the cognitive load on the decision process and therefore work to reduce the "what if I had picked something else" feeling.
- Following up with a confirmation email which reinforces the benefits of the product. Since users have short memories, they might focus more heavily on the hit to their wallets post-purchase and forget the benefits that brought them to the purchase decision in the first place.
- Assuring the customer of a simple and effective returns process (and perhaps also following up with this). This allows customers to ignore worries about the quality or effectiveness of the product, with the knowledge that it can be returned if it isn't fit for purpose (making the cost of a mistaken purchase minimal).
- Making sure the purchase aligns with the customer's goals in the first place. By presenting a full perspective on the product and guiding the customer to an appropriate item, you make it more likely that they will have a product that they will be satisfied with.
- Managing expectations is important too. If the customer believes the product to be something it isn't, then that's a problem. If you can outline the limitations of the product more fully, then the customer can take them into account before the purchase decision happens.
It's my opinion that buyer's remorse is inherently negative to the user experience, and that it is both possible and ethical to design to reduce it. However, as with all marketing, it's important to do so by solving the problems that cause the customer to react negatively, rather than designing around the problem by manipulating the customer.