It is a term used by media and seen by general public when talking about many unethical or illegal things like child pornography and drug marketplaces, so perhaps not the best thing to associate with in your marketing material.
So should we stop saying "hackers" because it is also a term used by media and seen by the general public when talking about many unethical or illegal things?
It is a fair point, and I don't necessarily agree with the perception or misuse of either word.
But you have to admit there are connotations and that leads to bias when you start talking to laypeople who don't understand the other meanings or usages of 'darknet' or 'hacker'. There may be better ways to communicate that won't immediately stop the general public in their tracks when they hear a particular 'tainted' word.
Who knows? The whole problem is that "darknet" is a label that doesn't mean anything definite, a buzzword but not a term.
You may say that a tool/network/protocol is decentralized and/or secure and/or anonymous and/or censorship-resistant and/or routed through Tor and/or doesn't leak identity and/or tamper-resistant and/or has plausible deniability etc etc and all these labels would mean something - "darknet" does not.
A "darknet messenger" might tick any set of these boxes, but the meaning is completely different depending on which of these labels apply.