When did the Soviet Union stop blaming the Entente for WW2?

I realize this is (years) late but another way to answer the question would be to consider the instructions given to the Western Communist parties and popular fronts, which can be gleaned from their official newspapers. I've never read those myself but I have seen it claimed that the American Communists called on their followers to do nothing to help the American war effort against Germany during the time the Molotov-Ribbentrop Pact was in effect (Aug 23, 1939 through June 21, 1941) which was followed by a very strong shift encouraging all American Communists and front groups to do their utmost to defeat the Germans pretty much immediately upon the beginning of Barbarossa.

I don't know if the Western Communist papers called for actual sabotage against Allied efforts against Germany or just passive resistance by declining work in war industries for the duration of the Pact. I don't know if the Western Communists actually committed sabotage against America, Britain, Canada, France, etc. before Barbarossa began. I don't imagine they would openly call for that, given the likelihood that they'd be arrested for that but I can well imagine them committing sabotage secretly on the orders of the Comintern representatives in their respective countries who were transmitting Moscow's desires. I do know that the Comintern had a change in focus in 1943 as a result of a request from the Western Allies to Stalin; even the name of the organization changed to Cominform to reflect that change in focus. I rather suspect the request to change the focus was made due to genuine concerns about the negative effects of the Comintern but I don't know the details.

An easy terminus post quem is October 31, 1939, which is when Molotov delivered a fine speech to the Supreme Soviet, containing the following lovely passage:

In any case, under the ‘ideological’ flag there has now been started a war of even greater dimensions and fraught with even greater danger for the peoples of Europe and of the whole world. But there is absolutely no justification for a war of this kind. One may accept or reject the ideology of Hitlerism as well as any other ideological system, that is a matter of political views. But everybody should understand that an ideology cannot be destroyed by force, that it cannot be eliminated by war. It is, therefore, not only senseless but criminal to wage such a war as a war for the ‘destruction of Hitlerism’ camouflaged as a fight for ‘democracy’.

The source is impeccable :)

Well, I own the Lesser Soviet Encyclopedia (1958) and it still blames the both sides.

Начавшись как агрессивная, империалистическая война между капиталистическими державами, Вторая мировая война для стран, противоборствующих Германии и её союзникам по фашистскому блоку, переросла впоследствии в справедливую, освободительную, антифашистскую войну. Решающим фактором, определившим превращение Второй мировой войны со стороны противников Германии в освободительную, справедливую войну, явилось вступление в неё Советского Союза, подвергнувшегося вероломному нападению гитлеровской Германии (...)

Started as aggressive, imperialistic war between capitalist powers, the Second World War, for the countries fighting against Germany and her fascist-alliance-mates, had developed later into a just, liberating, anti-fascist war. The decisive factor which determined the transformation of the Second World War from the side of the adversaries of Germany into liberating, just war was the entry into it of the Soviet Union who suffered the treacherous attack by the Hitlerist Germany (...)

The Great Soviet Encyclopedia (1971) changes the tone:

ВТОРАЯ МИРОВАЯ ВОЙНА 1939-1945, война, подготовленная силами междунар. империалистич. реакции и развязанная главными агрессивными гос-вами - фаш. Германией, фаш. Италией и милитаристской Японией. В. м. в., как и первая, возникла в силу действия закона неравномерности развития капиталистич. стран при империализме и явилась результатом резкого обострения межимпсриалистич. противоречий, борьбы за рынки сбыта, источники сырья, сферы влияния и приложения капиталов.

THE SECOND WORLD WAR 1939-1945, the war, prepared by the forces of the international imperialistic reaction and unleashed by the chief aggresssive powers - fascist Germany, fascist Italy and militaristic Japan. The Second World War, like the First one, arose due to the law of unevenness of the development of capitalist countries under imperialism and emerged as a result of contentions about sale markets, raw materials' sources, spheres of influence and application of capital.

So, the change happened between 1958 and 1971.