As a rule, historians seldom have direct influence on major political events and developments and German unification was not an exception to this rule. Most of the German historical establishment, exemplified by figures like Heinrich von Treitschke and Leopold von Ranke Wilhelm von Humboldt and Heinrich von Sybel, were National Liberals and Bismarck had little use for this political faction during the unification period. The National Liberals had called for the unification of Germany prior to 1871, but under more democratic principles. Although German liberals often espoused nationalistic sentiments that at times verged on the xenophobic, with also a healthy dose of anti-Catholicism, they went along with Bismarck's unification.

This is not to say that historians sat aside unification, many embraced it, German liberalism was quite nationalistic. What the bulk of the German historical profession did was emphasize historical interpretations and models that justified the unification of Germany under Prussia's aegis. Von Ranke, the prominent Humboldt University professor who imparted to the historical profession the mantra to write history "wie es eigentlich gewesen" (how things actually happened), privileged diplomatic and narrative history as the theoretical models for German historians to follow. Such a methodology meant that they crafted narratives that for all of their emphasis upon neutrality and objectivity, they sanctified the Prussian state as the apogee of realpolitik and level-headiness. Protestant historians in particular emphasized the destructive and divisive influence of Catholicism and foreign actors preventing the allegedly long-held dream of German unification from coming to fruition. There was a strong dissenting voice of Catholic historians from "Third Germany" that asserted a more nuanced reading of the past, but even there they took German cultural and national unity as a given. The methodology of German historians in the latter half of the nineteenth century seldom included data and sources outside of memoirs of major historical figures and archival records held by the state.

Although the above answer does take these historians to task for their shortcomings, it's also important not to minimize the accomplishments of these German historians. In many respects, these individuals were among the first to treat the historical discipline as just that, a discipline. Von Ranke emphasized training for historians as well as having a thorough primary source base before coming up with a thesis. Although they practiced a history that is decidedly old-fashioned and biased by the standards of 2014, this should not dim their accomplishments.

Edit: called von Ranke a liberal when I meant Wilhelm von Humboldt

Sources

Cramer, Kevin. The Thirty Years' War and German Memory in the Nineteenth Century. Lincoln: University of Nebraska Press, 2007.

Iggers, Georg G. The German Conception of History; The National Tradition of Historical Thought from Herder to the Present. Middletown, Conn: Wesleyan University Press, 1968.

_. Historiography in the Twentieth Century From Scientific Objectivity to the Postmodern Challenge, with a New Epilogue by the Author. Middletown, CT: Wesleyan University Press, 2005.

Mustafa, Sam A. The Long Ride of Major Von Schill: A Journey Through German History and Memory. Lanham: Rowman & Littlefield, 2008.