There is definitely an ethical component to nudging. A good and ethical nudge is designed to help someone make a decision they want to make themselves. A bad and unethical nudge makes it harder for a person to do what they want.
The rule of thumb for determining whether a nudge is “dark”, is by simply thinking whether you would be happy if your local supermarket made you do the same thing. Would you be happy if whenever you went grocery shopping, the manager put items she thinks you might like in your basket when—you weren’t looking? What if they made you walk through a tedious and deceptive maze every time you tried to leave?
Don’t nudge anyone with methods you don’t want to be nudged with yourself.