pp. Disrupting power by cutting or short-circuiting high-voltage electrical lines.
Other Forms
Five men have been charged by police in the eastern city of Ostrava with planning to cut electric lines to the Czech capital, North Moravia police criminologist Karel Trcka said. The so-called "darking" plot was uncovered while the suspects were being questioned in connection with other vandal-related blackouts in the region, about 300 kilometres east of Prague.

Police think the extremists planned to sever high-voltage wires leading to the Congress Centre, where NATO meetings are to be held, and to Prague's subway system…

The term "darking" was coined last year in the Czech Republic's Moravia region, where the practice of cutting high-voltage power lines and toppling power poles was first reported.
—“Czechs say anarchists planned NATO summit blackout,” Deutsche Presse-Agentur, November 14, 2002
2002 (earliest)
Darking, a new type of adrenalin activity involving short-circuiting of high voltage wires, has become rather popular in the Czech Republic, the daily Hospodarske noviny (HN) writes today.

HN says that several well organised groups have caused energy distribution companies problems in recent months by knocking out transfer stations for fun or in a fight against the monopoly energy producer CEZ, which experts say is a new adrenaline sport.

The practitioners themselves call the new type of sport darking. "It has been going on for about two years and we have registered tens of cases," Alena Holubcova, spokeswoman for the STE central Bohemian power distribution company told the paper. HN also reports that similar problems have been registered in north Moravia as well. "We have seen six cases recently," SME north Moravian energy distribution spokesman Pavel Novacek told HN.

The paper says that technicians who repair the transfer stations following darker attacks say that it is a miracle no one is hurt during the events, as the risk of electrocution is high.
—“'Darking' becoming more popular in Czech Republic,” CTK National News Wire, September 06, 2002
The word dark has been used as a verb in English for about 1,000 years. It has various senses, including "to become dark," "to obscure," and "to make dark." The preferred verb nowadays is darken, which is a good thing because it makes instances of this new sense of darking a bit easier to track down.

You'll notice that both the example and earliest citations refer to darking activities in the Czech Republic. So far this "game" appears limited to that country. However, in the more general sense of "losing electrical power," I saw a story in The Mirror from September 6, 2001 that talked about how in England some solar-powered parking meters weren't working because there wasn't enough sunshine to recharge them. The headline? "Darking Meters."