A court has blocked construction of a controversial truck stop on the outskirts of New Market, Md., a quaint Frederick County town that calls itself Maryland's "antiques capital."

The ruling was issued by Judge David L. Cahoon of Montgomery County, sitting in Frederick County Circuit Court. It overturned a decision by the Frederick County Board of Commissioners to reclassify a six-acre site from general commercial to a highway-service zone, the only category that would permit a truck stop.

"I was ecstatic . . . but a little surprised at the outcome," said Judy Stuart, who with her husband John owns one of the town's 40 antique shops and was a leader of the opposition. "I thought it was wonderful," said Edgar W. Rossig Jr., head of a citizen group that fought the project.

They contended the truck stop would spoil the atmosphere that permits the antiques businesses to thrive there.

The decision came a month after the Cardon Oil Co. of Mesa, Ariz., began grading the site for the $1.25 million truck facility off I-70.

In his ruling, Judge Cahoon said any "significant changes" in the character of the area that might justify the construction of the truck stop must have occurred after the county updated its overall zoning map in 1977. In fact, Cahoon found, all such changes actually had occurred prior to 1977, so the rezoning was improper.

The county and the oil company have 30 days to appeal. County Attorney Lawrence Speelman said he will discuss the issue with the commissioners. John C. Murphy, a Baltimore lawyer representing the oil company, said a decision will be made later.