A B/N is an Adhoc network (mainly Wifi or LAN) that is not directly connected to the internet, used for sharing files.
Networks like this could span over large areas, with the use of repeaters, cantenna's or dedicated network cables, but are mainly very local.
They consist of a storage device connected to a router, giving access to other users (eg neighbors, classmates, etc) to read (but not write or delete) data from the drive.
Once files are downloaded, they are put on a storage device.
They are shared over a separate net to multiple users.
They can be password protected networks, that use custom P2P programs (with access codes or passwords) for the transfer of files, but also could use the standard Windows Adhoc network shared file system to copy or stream files over the network.
Networks can quickly clone themselves by cloning the harddrive, and installing it with a router on another location,creating a second network.
Black networks often are created to diminish suspicion of the RIAA or other law organizations.
Quite often they are found in apartment complexes, boarding schools, or campusses,and amongst internet techies or nerds.
They are also known as AdHoc networks, or intranet networks.
Not all uses have to be illegal,
They can be used to play online games with friends, or provide a faster way to share legal files amongst many people.
2- Multiple Black Networks can share data with eachother, eg: B/N 1 shares software, while B/N 2 shares video's; then cross share the data with eachother.
3- Files can be shared to multiple clients without those clients to need to be online (once the files are uploaded to the Black Network)
4- Files can be backed up across Black Networks in case of one network's harddrive failure, creating clones could provide a backbone for other Black networks.
5- WLAN,Wifi, or LAN networks provide faster access times and download rates than the internet,and generally are free (save the cost of electricity and the purchase of the hardware).