The turning point was Operation Cast Lead, the war in Gaza that began at the end of 2008 and ended three weeks later with a Palestinian death toll of around 1,400.
In a statement issued when he was first imprisoned, Blanc said: "The wave of aggressive militarism that swept the country then, the expressions of mutual hatred, and the vacuous talk about stamping out terror and creating a deterrent effect were the primary trigger for my refusal."
The government, he said, was "not interested in finding a solution to the existing situation, but rather in preserving it... We will talk of deterrence, we will kill some terrorist, we will lose some civilians on both sides, and we will prepare the ground for a new generation full of hatred on both sides... We, as citizens and human beings, have a moral duty to refuse to participate in this cynical game."
In an interview with the Guardian, he says: "The war going on in this country for more than 60 years could have ended a long time ago. But both sides are giving into extremists and fundamentalists. The occupation was supposed to be temporary, but now no one speaks of it ending."
The Israeli state, he adds, keeps people "under our control" without democratic rights. Palestinians are subject to "collective punishment" for the actions of a few.
Most Israelis grow up knowing that compulsory military service – three years for boys, two for girls – lies at the end of their school days. "Going to the army" is a deeply ingrained, collective experience in Israeli national identity.
For some, it's an eagerly-anticipated patriotic duty; for others, a rite of passage; for a few, a difficult moral dilemma. But it is rare for people to refuse on grounds of conscience. Blanc says that since November, he has been the only conscientious objector among the 300-400 inmates in Military Prison Number 6.
Most of his friends have come to accept his position – "we had the arguments a long time ago" – and some who are currently serving as combat soldiers now say they admire it. His parents, despite some anxiety, are supportive.
Blanc rejected the option taken by some objectors of claiming a medical condition that would exempt him from military service. "I didn't want to lie. This is a point of principle."
Neither could he seek exemption of the grounds of pacifism. "The army has a narrow definition of pacifism -- someone who would never apply force in any circumstances. The [IDF's] conscience committee asks tough questions, and I would not be able to say never. I think force should be used rarely, but it can't be completely ruled out."
Blanc is willing to undertake national service in lieu of the compulsory stint in the army, but thus far the military has refused to countenance this.
In a statement, the IDF said it could not comment on Blanc's specific case, but conscription was a result of Israel's security situation. Although there were limited grounds for exemption, those called up were "fully aware of their responsibilities towards the military and the consequences for failing" to carry them out.
Blanc hesitates when asked if he would describe himself as a patriot. "I feel a strong connection to this country, and I'm proud of it in many ways. But I have an aversion to nationalism."
Prison life has taken some adjustment. Blanc, who shares a tent with around 20 other prisoners, is woken for roll call around 5am and works eight hours a day in the kitchen. The inmates, who wear surplus US military uniforms, can make calls on a public phone but are forbidden to keep their mobiles. There is a prison library, but no gym.
"I have no idea how long this will go on for," said Blanc. "The bad scenario is that I will be put in front of a military court and sentenced to something like a year in prison. The better scenario is that they'll get tired of this, and will let me do national service instead."
It is hard for Blanc to see beyond the game of cat-and-mouse in which he and the IDF are currently engaged, but he says: "I don't want to deal with politics and conflict all my life." He would like to study science or technology at university.
He brushes aside a suggestion that his current stance could harm his future prospects. "I'm proud of what I'm doing. I may have caused some damage to my future, but it's minor compared to the principle at stake."