The role of the German immigrants in Missouri was rich and complex even before the war, but especially so during the war. In my hometown of Boonville, a Home Guard unit comprised mostly of German immigrants received word of an impending attack by pro-southern forces in early autumn, 1861. This word was brought to them by runaway slaves. The commander of the Home Guard unit, named Eppstein, reportedly armed and even uniformed the runaway slaves, who served as combatants during the rebel attack. This might be one of the earliest cases of blacks being armed and uniformed and serving in combat roles during the war.
However, instances such as this didn't mean that German immigrants were necessarily integrationist. Eliminating slavery might have been one thing, but segregation was alive and well for a very long time around here, as it was elsewhere, too. Plenty of people with German surnames continued to be extremely prejudiced for generations after the war. This didn't make them unique among immigrant groups by any means.
The irony is that Germans, Irish and other immigrant groups were often victims, themselves, of cultural biases--even as some of them maintained their prejudices against blacks.