Oh noes my friend.. Its not about negative energy, but background (void) temperature. The pair of particles (matter / anti-matter) spawn, cooling down the vacoom. Once they anihilate again, they change to energy (various photon emisions) increasing background temperature again. Thats why empty space is not at 0 kelvin. Every time such particle is absorbed by blackhole, it just increases it energy (and possibily mass). Not sure where that evaporation comes from ;)