Several historic trends came together to produce the election of Salvador Jorge Blanco to the presidency of the Dominican Republic last month, but his success in office is likely to depend on something he cannot control: the international price of sugar.

The energetic attorney defeated two legendary leaders: Joaquin Balaguer, 75, last of the old right-wing strongmen, who ruled the island nation for most of the period between 1966 to 1978; and Juan Bosch, 73, the grand old man of the left, who still blames the United States for the military coup that ousted him from the presidency in 1963.

Jorge Blanco, 55, and his Dominican Revolutionary Party (PRD by its Spanish initials) took the presidency and both houses of the legislature on May 16 and control of most local governments. The Army, which has had a hand in every election since the country was founded, stayed out of it this time; the oligarchy that historically kept its money out of the country except to pay off the Army was either silent or behind Jorge Blanco.

Leftists campaigned for four years, the first time they did not have to risk gunfire to do it, and their various splinters including Bosch's took less than 13 percent of the total vote.

"Jorge Blanco now has a tremendous mandate and the power to put his program into effect," said Rafael Fafa Tavares of the United Leftist Front, a coalition of 10 radical parties. "People still have faith that something can be done. It will be a disaster for them the PRD when they see that nothing can."

Partly because of the rock-bottom international price of sugar, economic conditions in the Dominican Republic are approaching catastrophe. Unemployment is officially 30 percent, with another 20 percent working only sporadically. The airy castle-size houses of the landed rich are crowded by the palm and branch shacks of the rural poor who have swarmed into the cities in search of work.

The pressures on Jorge Blanco to do something visible about the economy during his four-year term are tremendous. The Army, whose most "interventionist" generals were purged over the last four years by ex-president Antonio Guzman, retains a tradition of ousting inept governments. Businessmen want a stable investment climate, which Jorge Blanco has promised, and are wary of his social democratic politics and his party's alliance with the Socialist International.

But most of the traditional solutions to grinding poverty and the restlessness of the urban unemployed are long-term ones: teacher and technical training colleges, school and factory construction, rural development projects. To get going, the Dominican Republic stands to receive $40 million in economic aid under President Reagan's Caribbean Basin Initiative, which is being reshaped in Congress, plus $10.25 million in military assistance proposed for 1983.

One of Jorge Blanco's first acts as president will probably be to close a three-year International Monetary Fund deal, reportedly for $600 million, braving the outcry over loss of sovereignty that Balaguer supporters will surely raise.

Land reform is next. "The Balaguer people will be shouting, 'communism, communism,' about every reform he tries," a respected social scientist predicted. "We are all worried that the State Department will listen."