Is the anonymous usage of Darknet Markets (DNMs) the goal of Monero developers?
No, it is just a possible usage that Monero developers neither promote nor advise against.
From the very earliest days of Monero's existence we've been quite insistent on not writing any use-cases for Monero. Our baseline use-case for Monero is somebody purchasing something, like a religious text, that puts their lives at risk due to it being banned by the government. If we treat every Monero user's privacy as just as important as that hypothetical person, we will be able to successfully improve Monero's privacy and protect all its users.
Of course, a tool that provides that level of privacy can also be used for all manner of purposes, both amazing and nefarious, both legal and illegal, both innocent and far-less-so. Let me be as clear as possible: our focus is, and always will be, on building out and enhancing Monero and all its tools, regardless of the use-cases of individual users.