As cunning as Ganelon himself, Blancandrin is the man of the hour when he formulates the plan to get the Franks out of Spain. Persuading the Saragossans is easy; it takes more skill to sell the plan to Charlemagne and Ganelon, the envoy. But Blancandrin is up to it.
During their private ride, he pushes all of Ganelon's buttons. Lagging behind to be alone with him (28.368), he starts by flattering Charlemagne, whom Ganelon seems to sincerely admires, and then calls Roland a maniac, slyly leading Ganelon from ideas of private revenge to something that looks a lot more like public treason (30).
After all, it's during their first conversation that Ganelon decides to have Roland killed (31) and in their second conversation, after Ganelon has spoken to Marsile, "they negotiate the wrongful act of treachery" (38.511). In other words, they figure out the deets: the where (Roncevaux), when (soon), the who (Roland and the rearguard), and the how (army massacre). How responsible is Blancandrin's cleverness in causing Roncevaux? Would Ganelon have come up with this on his own? Unclear.
Blancandrin's motivations start out blameless—securing peace in Spain—but he seizes on the idea of killing Roland and the rearguard as soon as he sees an opportunity in Ganelon's festering hate. In some sense, this might make Blancandrin's deceit more patriotic and excusable than Ganelon's, who kills his own stepson and ruins the military fortunes of the Franks. Like Ganelon, he remains an ambiguous guy.