The OUN […] similarly introduced a range of fascist principles and rituals, which became obligatory for all members of the movement, and which, after the establishment of the Ukrainian state, were to become obligatory for all citizens. The red-and-black flag, which symbolizes blood and soil (Ger. Blut und Boden), was one of them. These colors referred to the racist and proto-fascist German blood and soil ideology, which suggests the inseparability of a people (Rasse or Volk) and their “living space” (Lebensraum) as well as an attraction to the “soil,” which acquired spiritual and mythological connotations.

(Source.)