A black dwarf used to be a type of dark, lifeless dwarf star. These were white dwarfs that had cooled down immensely over vast timescales, to the point where they emitted no measurable light or heat and appeared black. The cooling process would require many billions of years to complete.
The exogorth named Sy-O came into existence close to one of these stars, which had a core made of iron, and it sustained itself by consuming the star's remaining energy.