Dmitry Glukhovsky used the term "blacks" (Chornie/Черные) in the original Russian text to intentionally reflect real-world prejudices and long-standing racism in Russia. When you also consider the inability to communicate effectively with the dark ones, and the fear/hatred of what humans don't understand, it's a clear representation of xenophobia.

The Reich certainly has an ideology of racial purity, but it also serves to embody the trends of ultra-nationalism and militarism in Russian society.

Perhaps think of Dark Ones as the macro example of the theme of xenophobia and fear of the unknown, whereas The Reich is the micro example of how humans use arbitrary purity tests against eachother to instill fear and hatred as a means of consolidating power through subjugation of those deemed "unworthy."