Darknet is an area of the internet that we do not have anyway of getting into it but the hackers, criminal of all types have found a way in and use it to post there criminal activity.
Apparently, Jay is of different opinion on a “Darknet” concept 
Jokes aside, I2P is already very usable, have a look at the following tutorial:
https://www.youtube.com/watch?v=kz4q06jdlCE
I used it, just to see how it works, and was able to procure some ebooks and (classical) music mp3s.
If nothing else – one may want to set up a copy of their legitimate “clearnet” website on one (or more) of the Darknets, just in case…