As in: What were the implications of this accusation that so offended the accused ones? It’s the only reaction to his speech the transcript of his speech mentions at all.
These "negroes" in question were French occupation soldiers.
Among many groups, mostly right wing, in the Weimar Republic, there was the notion of a "black shame" - that France didn't deem Germany (or more precisely the Rhineland) worthy of being occupied by white soldiers, instead sending their colonial troops. These colonial troops were accused of widespread rape and looting - which did happen, but most of the commotion was just propaganda, which was mostly opposed by communists and the left-wing USPD (the "Unabhängige Sozialdemokratische Partei Deutschlands" - a splinter group of the Social Democratic Party).
Of course, the underlying debate was one of racism. It was even carried out on the parliament floor - noteworthy would be one debate in 1920 for example, where Luise Zietz, a member of the USPD, stated that militarism was at the heart of the alleged crimes of the black soldiers in the Rhineland, because German soldiers had behaved just the same in Russia, France and even China during the boxer rebellion. Her remark, that the blacks are "capable of culture" earned her the suggestion that she should "paint herself (black) then". Meanwhile, this speech was so upsetting to the parliamentarians, that most left the plenary hall during it. This display of opposition gives a fine idea in what context the debate about "negroes in Germany" was led during the time.
In 1921, Otto Thomas, one of the leading bolshevist voices in the communist party, called upon the nationalists in the communist newspaper "Neue Zeitung" (which also was published in Munich, by the way) to stand together with the communists against the foreign oppressors, specifically calling out the black colonial troops as the ones to rise up against.
Eugen Leviné now accusing the government of leading "negroes" into battle against the Bavarian Soviet Republic also goes to show that the communists were not all safe from the racist notions of the time either. And it was of course deeply upsetting since it accused the state (i.e. the court and the representatives of the government) of both collaborating with "inferiors" as well as resorting to external help of the occupiers to defeat the righteous revolution, who would otherwise have prevailed thanks to the support of the masses. So the politial angle was to discredit the counter-revolution, while the social angle was a racist one - whether Leviné himself was really racist, we do not know, but if he wanted to elicit a reaction, it was definitely the way to go at the time.
Sources: If you read German, "Von Wilden aller Rassen niedergemetzelt" - Die Diskussion um die Verwendung von Kolonialtruppen in Europa zwischen Rassismus, Kolonial- und Militärpolitik (1914-1930) by Christian Koller is, in my oppinion, an excellent foray into this topic.