The Black Light is the counterpart of the [[White Light]], where one can convert to the evil Goddess [[Libila]]. To reach it, head North West from the [[White Light]], which can be seen from the whole southern part of wurm, for example from [[Newtown]].
NOTE - If you join Libila you automatically become [[Horde of the Summoned]] aka 'Blacklighter (short: bl'er)' (HOTS) and cannot spawn or go anywhere near [[Newtown]] (NT).