A truly communist country is impossible for a couple of reasons according to Marx's own logic. Firstly, Marx thinks that the revolution will be international and will happen all at once. The revolution in one part of the world will spark revolution in the other parts and in a few weeks or months all the bourgeois governments will be replaced by a global dictatorship of the proletariat. Communism cannot therefore exist in isolation, everyone has to go socialist at the same time. Marxists need this because they realise that the bourgeois order will not just die, they will fight for their survival and way of life.

Secondly, a true communist society would be one without a state -- the state and the nation will cease to exist. Marx thinks that the state works as an instrument of class oppression. It is in itself neutral but is used as a tool by which ever class happens to be in power at the time. the Bourgeoisie use the state to oppress the workers and ensure they they maintain control of the means of production. Think how often the police are brought in to mediate industrial disputes and so on. When the workers take over the state, during the dictatorship of the proletariat, they will also use the state as a means of oppression. They will destroy the remnants of all other classes. When only one class exists, the workers, and nothing separates them materially then what is the point of the state? There is no one left to oppress and everyone lives in brotherhood. The state therefore withers away, it is imply not required any more.

Nowhere has ever reached this place. To take the soviet example, there was no global revolution. The Soviets therefore had to continually defend themselves against the forces of capitalism. The state could not wither away while there was still a need for collective defence While they made great strides towards creating an egalitarian society privileged endured and new classes emerged.

Please note that my explanation argues basically from Marx in a vacuum. Socialist/Communist leaders did believe passionately in the idea of global revolution (Che Guevara's adventures in South America and Cuban intervention in Angola come to mind as examples where a socialist state sponsored other left wing governments). I am attempting to explain the process by which Marx tries to get us to the idea of global revolution and why it didn't work out.

Keep in mind that what we called "Communist countries" did call themselves "socialist countries," i.e., the Union of Soviet Socialist Republics, the Czechoslovak Socialist Republic, and so on. They did so for the reason that you name: according to their own belief system, a state could not be communist.

In any case, historians still use the term "Communist state" to refer to states that were controlled by the Communist Party, with a capital 'C.' The Communist Party's goal was to bring about the communist revolution, or at least this was the goal of a significant number of its members.

Despite the failures of the Communist nations to bring about the revolution that the various Communist Parties envisioned, many Communist leaders, notably some of the most powerful politicians in the Soviet Union, did strive to reach this goal. Historians have shown considerable interest in the ideological components of Soviet history, and they have demonstrated that faith in the inevitability of socialist transformation and revolution played a key role in the decision-making process through most of the Soviet Union's history. You will not find historians (at least historians today) making the same argument that /u/azdac7 has made here, i.e., that we can dismiss the goals of Communist leaders because the communist revolution envisioned by Marx never occurred. It matters what these historical figures believed, not whether we think their beliefs were "truly" Marxist (and therefore "communist") according to our interpretation of Marx.