I'll blame it on Germany. That may sound like a joke, but it's not meant that way.
The Soviets took Eastern Europe because they feared being invaded from the west. Germany had just done this too them early in WWII. The German invasion (along with that of Napoleon way back in the 1810s) made the Soviets really afraid of the West.
Because they feared the West, they took Eastern Europe. Because they did that, the US feared them and the Cold War was on. So, in a way, it was the German invasion of the Soviet Union in 1941 that could be blamed for the Cold War. Although the United States and the Soviet Union were obviously the two main culprits, it was the Germans who created the Soviet mistrust of Western Europe and its allies.
Blame could probably be shared equally between the United States and the Soviet Union. Primary responsibility would be with the Soviets, who refused to withdraw from Eastern Europe, and planned to continue their expansion. Winston Churchill warned that "an iron curtain" had followed over Eastern Europe. In the United States, there was a second Red Scare, and the belief that the Soviets would attempt to take over here; primarily through subversion. Propaganda ran high on both sides. When the Soviets developed an Atomic Bomb, the United States went one step further and developed the Hydrogen Bomb, which the Soviets quickly duplicated. President Truman's policy of Containment--that is to prevent communism from spreading beyond the areas which it then occupied--was also a factor in the escalating tension. In the end, the entire world was separated into the two camps, East and West. Chances are it would not have happened had the Soviets not been so aggressive; yet the propaganda in the U.S. was also a substantial factor.