To me, this poses an interesting question;
What is a dark pattern?
Are they something that 'tricks the user'? encourages them to act against their own interest?
or are they something that happens in the shadows of the subconscious? Perhaps some encourage the user to act in their interest. Just subconsciously.
I feel this might be too broad as many psychological effects are not consciously observed by the general user. So let's assume it means a pattern encouraging behaviour not in the user's best interest.
Then there is a further question:
What if you are encouraging something that is against the original wishes of a user but in their interest? (some might debate this concept itself but I'll leave that alone for now).
Ethics is philosphy....
...Therefore debate is it's oxygen.
Ethics is something that has been debated for thousands of years. Being a branch of philosophy it is inherently changing and hazy in some areas. For example a stream of ethics - Consequentialism - hold that the consequences is the basis of moral judgement. This could lend support to the question above. What if, despite being against the wishes of the user, the final outcome was in their interest. Is this ethical?
Other streams claim that contentment or happiness should be the measure. But is that over a short term (where a decision might feel right) or long term (where it might prove to be wrong)? Not to mention the social aspect where you could consider the ethics of when the pleasure of one impacts on another. Pursuit of profit may fall in this category too.
Another stream states that moral judgement should be based on telling the truth (deontology). This might seem clearer. However, what is truth? If we revealed all the mechanisms and facets at play it would inherently increase complexity. Ironically being more unfair to users. How many times do you make judgements for the user? Telling the truth through a lens.
The line is not only grey. It is broad and changing. This is where 'truth', debate or exposure can allow a person or society to decide whether they are willing to accept the practice. One essence of culture is how we frame and react to these situations. (A whole different topic I will skip right now). Personally I consider some dark patterns to be clever, some I'm uncomfortable with and some make me outright mad (hello RyanAir and Fox News...).
Perhaps the aspect that provides the best summary is this quote from Immanual Kant:
Nothing in the world—indeed nothing even beyond the world—can possibly be conceived which could be called good without qualification except a good will.
Therefore you may act within presently defined laws, but without a basis of good will it may not be ethical.
On a personal note, I believe that if a business acts with good will. Holding that long-term goal over short term gains, they will be more successful. There is some strong evidence for this. (Plus, I want it to be so :) )