Rockingham Park thoroughbred track, a golden oldie, is dark on Tuesdays, meaning no racing scheduled. Yesterday was a very dark Tuesday, and many more dark days are ahead. Fire virtually destroyed the place in Salem, N.H. yesterday morning.

All that remained of the four-story, 13,000-seat grandstand after the fire, which broke out while several horses and riders were out for exercise runs, was a pattern of twisted steel beams. The clubhouse and paddock areas were gutted.

More than 500 people were put out of work, and a vital source of revenue to the state of New Hampshire shut off, with only 21 days of a 68-day meeting completed.

But the blaze spared 1,400 thoroughbreds in stables a quarter-mile away, no injuries were reported, and there is hope of reopening quickely -- target date Friday, Aug. 8, an official said -- with portable bleachers or tents to replace the grandstand.

One major problem, through: $2-$3 million worth of computerized equipment destroyed. But offers of help flocked in from tracks as far away Florida. As a Rockingham spokesman said, "I guess everybody looks at it as, 'Well it could be me next'" . . .

Grumbling all the way, the New York Knicks have accepted Seattle's No. 1 draft pick in 1981 as "reverse" compensation for having "overcompensated" the SuperSonics, courtesy of the NBA and judiciary powers-that-be, in that 1978 free-agent signing of Marvin Webster. The NBA Players Association might reluctantly let it with that too, say boss Larry Fleisher . . .

Al McGuire has moved up from the 25-person membership to chairman of the President's Council on Physical Fitness, succeeding New Mexico Gov. Jerry Apodaca . . . Tennis' Martina Navratilova has decided to live full time in the 27-room Charlottesville, Va., mansion she bought early this year. She put her North Dallas home up for sale and already has transferred the headquarters of her corporation to Virginia. It's named Brat Inc. Huh? . . . . Another idiot throwing a beer bottle out of a passing car has cause tragedy, loss of an eye and disfiguration to an English soccer fan visiting Florida. Neil Bolton, 22, was the victim outside a tavern near the stadium after watching his hometown side, Nottingham Forest, play NASL's Tampa Bay Rowdies. He'd saved a year for the trip; has flown home two weeks early . . .

A happier race track item: Charles Town raises purses effective tonight, for a second time since February. The $100,000 boost for the meeeting, says G.M. John Battaglia, "further reflects management's optimistic view of Charles Town's future" . . .

Larry Little, the Miami Dolphins' all-timer at offensive guard, has ended a brief retirement: the 14-year vet settling into a backup role.