Germany got the blame because it lost WW1. At Versailles, to get reparations all the blame was laid on Germany's doorstep: the War Guilt Clause. http://en.wikipedia.org/wiki/War_Guilt_Clause

Clearly in hindsight this was more a political decision than a real historical argument. So who was to blame?

I can't blame the Austrians for going to war with Serbia. The Serbs killed the Austrian Archduke (and heir) after all and refused to hand over the masterminds behind the assassination. [Just imagine if the President-elect got assassinated while visiting a foreign country.] The Serbs really are the ones most to be blamed.

Having said that, the Austrian ultimatum to Serbia was harsh and designed to provoke a war. At this point, Serbia had a choice: swallow its pride and accept the ultimatum or go to war. Personally, I can understand the Serbian government going to war over the ultimatum.

So the next question is who is blame for the wider war? Some argue that Germany is to blame because it gave the Austrians a blank cheque: Germany told Austria that Germany would back Austria no matter what. I'm not so sure Germany is to blame for backing up an ally after its heir is killed. [I would certainly hope that Britain back the US if the President-elect got assassinated and we wanted justice.]

Personally, I think the Russians should be blamed for the wider war. Even if they had a pan-Slav ideology, the Russians should not have backed up Serbia (the party clearly in the wrong). Russia didn't have to mobilize its troops to protect the "bad guys." If Russia didn't mobilize, then Germany would not have mobilized and the war would have remained a minor war between the Kingdom of Serbia and the Austro-Hungarian Empire.