The African prescence in Germany before WWI was very small. During WWI, when French troops, including black troops from African French colonies, occupied parts of the Rheinland. As with occupying troops of any army, they left behind babies conceived with their local lovers etc.

You can read about that generation of afro-germans and how they were treated by the Nazi's here.

http://en.wikipedia.org/wiki/Rhineland_Bastards

In WWII, Germnany was occupied by American soldiers, including American "black" soldiers. Once again, numerous children were born from US troops having relationships with local Germans -- the mixed race babies were easily spotted in a mostly white european country. (They are still being born wherever one country has troops stationed in another country -- the mixed race ones especially stand out ) Babies from that generation are called "brown babies"

http://www.history.ucsb.edu/faculty/marcuse/classes/133c/133cproj/08proj/Fehrenbach2005Capozzoli08z.htm

or

http://www.spiegel.de/international/spiegel/0,1518,456835,00.html

or just goole search for "brown babies" but these are recent historical events.

As far as slavery is concerned, there were a considerable number of German settlers in colonial America. In the simplified American history we learn in most schools, we learn little about them, but especially from New York through South Carolina, there were considerable numbers of them. "he first Germans to arrive in the New World settled in the English colony of Jamestown, Virginia, in 1608. It wasn’t until the 1680s, however, that significant numbers arrived, settling primarily in New York and Pennsylvania." from
http://en.wikipedia.org/wiki/German-American
and much more here:
http://en.wikipedia.org/wiki/German-American#17th_century

Which mentions that "in 1790 German Americans were about 9% of the white population"and "By 1775, Germans constituted about one-third of the population of Pennsylvania. "

They were white --- and some, especially from these early migrations,took advantage of the institution of slavery, especially in the states like South Carolina where slavery was the standard method of acquiring agricultural labor - while certain religious groups were famously against it.

German Americans, like African Americans were here from the very beginnings of American history, and like African Americans have participated in every facet of "American" culture -- and that includes the "peculiar institution" of slavery.