the short-lived "German colonial era" has perished not too long ago in the "rising years" during 1921-36. Many former colonists, then returnees and doubtful of the benefit of a democratic society in comparison to the old regimen, have supported the National Conservatives (with the objective of reestablishing monarchy) and/or the Nazi party.
Even when not counting those (and the "fan service" going with it), there has been a continuous history of overseas slave trading in Electoral Brandenburg>The Prussian Kingdom>The German Empire for 300 years (a significant part of the riches of the Hohenzollern dynasty and of the respective state was generated by colonial trading), in the "highly civilized" 19th century each moderately sized town in Germany had at least one merchant dealing with "colonial goods" and zoological gardens used to showcase live human beings. No, racistic resentment itself has pre-existed in society long before the Nazis. They have exploited it, sure, and indeed, racism is core of Nazi ideology.
The thing is that there was no Black person to blame for the loss of the Great War of 1914-18. at least I doubt there was many Black Jewish or Black Bolshevik people around. and even if so, for people from the colonies it was factually impossible to legally become a German citizen (German courts until today are citing the very same laws when it comes to denying citizenship to Black people with a German Grandfather).
As a side-note, discrimination of Black People still happened, just in another way. as anti-Americanism was immanent in Nazi Germany, the roaring "American lifestyle" of the 20s and 30s was identified and subsequently rejected as "non-German"- in particular the modern swingy and jazzy trends in music, dubbed "ape music".
as a second side note: the Berlin Olympic Games of 1936, most successful from the POV of Nazi leaders with regards to the overall number of gold medals won. unfortunately for them, it was an African American who stole the show. 4-time-gold-medalist Jesse Owens. Of course, the games have been essentially instrumentalized by the Nazis in order to create a distorted picture of the reality in the country and show it to the media of the world, but, non-the-less, Jesse Owens has been granted due honors by the officials, by German media and by Nazi leaders. (he should be considered the exception to the rule).
Yes. There was a small number of people with African ancestry in Germany during the Nazi era. Most were the children of African-American troops who had been stationed in the area during World War I. They were an early target of Nazi race laws. Even before the Nazis came into power, they were discriminated against, unable to hold certain jobs, go to university, and were socially ostracized. When the Nazis came into power, the situation got much worse. Many were subjected to involuntary sterilization, though there was never specifically an order to round them all up and kill them all. Nevertheless, many were killed by the Nazis, many were subject to medical experiments, many were badly persecuted, tortured, and imprisoned. Some were sent to concentration camps. Black POWs were mistreated and many were sent to concentration camps rather than the much more humane POW camps.