Looks like the black stuff is carbonized wallpaper to me.
Notice how the "tree" seems to originate at the strap holding the wire. The wire insulation appears to be damaged by the (improper) strap, which is now charged with high voltage. The voltage is leaking to ground (the T-bar ceiling grid) via any conductive path it can find, in this case conductive threads and surface dirt on the wallpaper.
When the "tree" grows enough to bridge between the 2 wires or to ground, a fire will probably result....
