"Blackout!" shrieked the two-inch headline in the New York Daily News today. Local television was full of hand-wringing merchants and blaring fire engines. Even The New York Times, perhaps seeking relief from wars and natural disasters elsewhere, devoted half its front page, more than two inside pages, six stories, nine pictures and two maps to the story.

But among common New Yorkers it was hard to tell anything was going on. Rush-hour traffic sped through the area where the underground electrical fire blacked out a 12-block area of the garment district Wednesday and today. Secretaries in running shoes and Walkmans made a beeline home this evening with hardly a sideways glance. Soft-pretzel stands were out in force despite afternoon showers that turned into heavy rains tonight, shutting down half the city's subway system and knocking out electricity to several thousand homes.

Even the criminals, so unruly during the 1977 blackout, were nowhere to be seen today. No incidents of looting were reported, and Assistant Police Chief Michael McNulty, overseeing a contingent of 600 officers called out to guard the area under the glare of lights powered by 62 mobile generators, said none was expected.

"This brings out the best in criminals," said Mayor Edward I. Koch, in a Big Apple-boosting mood. "They cease criminal activity when we're under stress, apparently."

Demonstrators went about their business. A placard-carrying group circled in front of a Japanese bank a few blocks from the blackout area, shouting angry slogans about loans to Chile.

Macy's and Gimbels closed their doors, and the Dry Dock Savings Bank removed half a million dollars from a vault underground where Wednesday's fire blazed, but a few of the smaller merchants, with New Yorkers' nonchalance, operated anyway.

Yesterday's fire, caused by a broken water main that flooded a Consolidated Edison electrical substation, caused no injuries. But merchants who operate in the blacked-out area between 6th and 7th avenues from 30th to 42d streets could lose hundreds of thousands of dollars in business before power is fully restored Monday.

In the garment district, this was one of five "market weeks" during the year, when thousands of out-of-town buyers come to place orders. Other businesses, including restaurants, banks and hotels, also suffered.

Steve Harris, owner of Celebrity Travel Agency at 501 7th Ave., sat glumly in the un-air-conditioned dark, his flashlight reflecting eerily over the brochures on the wall. Thirteen of his 14 employes came to work today, operating by flashlight and battery-powered lamps.

"If I hadn't come in, I would have lost five tickets to the Orient and $2,000 in commissions," Harris said. Nonetheless, he added, "I'm suing Con Ed and I'm suing New York City. This will probably cost me $25,000." Meanwhile, crews worked feverishly to pump out water from underground. Thirty-five teams of Con Ed employes began to splice underground cables to bring power back into the area.

Although damage was not visible from the street, the 15-hour fire generated heat of about 1,000 degrees in the underground vault. City crews planned to enter it to check for potential contamination by polychlorinated biphenyls, a cancer-causing substance known to have contaminated at least 150 gallons of oil used in one of the seven transformers that burned in the fire.

Inspectors examined buildings for structural damage. The Dry Dock Savings Bank, at the corner of 38th Street and 7th Avenue, the site of the power substation, suffered basement damage.

But the bank expected to reopen Friday, which was good news to the slight, white-haired gentleman standing outside under a large black umbrella. David Cohen, 75, a stockbroker, had walked from his home on 23d Street to see if he would be able to withdraw some money. He was unfazed by all the excitement. "It's one of these things you expect in the civilized world," he shrugged, "along with muggings, robberies and rapes." He smiled pleasantly and walked off.