Definition:
Dark Launch is a process where software is gradually or stealthily released to consumers in order to get user feedback and test performance . A go-live strategy during which code implementing new features is released to a set of the production environment however isn’t visibly, or only partially, activated. The code is exercised, however, in a very production setting while not users being tuned in to it.
Further Reading:
Book: The DevOps Handbook by Gene Kim, Jez Humble,Patrick Debois.