Forgive my ignorance but weren't they essentially a socialist state with certain degree of dictatorship factored in? (same applies to China and North Korea)
Communism and socialism are not exclusionary concepts. Communism is a form of socialism.
The year 1917 saw two revolutions in Russia. The February Revolution (actually in March, but Russia was still on the Julian Calendar) overthrew the imperial regime. The October (November) Revolution was launched by the Bolsheviks led by Lenin. The Bolsheviks were a breakaway faction of the marxist Russian Social Democratic Labour Party. The Bolsheviks officially names themselves the Communist Party of the Soviet Union in 1912. This party ruled the USSR until its dissolution.
So, to answer your question, the USSR was always considered Communist because that was the name of the political system promoted by the party which founded it. See Marx's Communist Manifesto for an earlier appearance of the term.