What led the Soviets to blockade West Berlin?
West Berlin was part of Eastern Germany
Berlin had been cut into two parts and the Soviets were occupying the eastern part of Germany and the blockade was thus meant to force the West leave West Berlin to the soviets.
Berlin the capital of Germany was divided between the four allied nations according to a treaty conducted before the end of World War II. Germany as a whole was also divided between the four allied nations after the war. The Division of the greater part of Germany left Berlin surrounded by the part of Germany controlled by the soviets by treaty.
After the war there was suppose to be an election determining the future of Germany. The election was held in the three parts controlled by France, England and the United States resulting in a western style democracy. The Soviet's refused to allow Eastern German the part controlled by them to participate in the election resulting in a communistic government under the control of Soviet Russia.
Western Germany considered Berlin to be its capital thus laying claim to all of Germany. Also many East Germans defected to the West by traveling to Berlin. The Communistic government wanted to stop the bleeding of their population to the West and to gain control of the Capital giving legitimacy to the Communist claim of all of Eastern Germany.
The blockade was an attempt to take over all of Berlin. When the blockade failed an elaborate barrier and wall system was built to keep the people under communistic control out of West Berlin.