After the October Revolution, when the Soviet Union established its armed forces, it largely followed the Russian tradition. The clothes were also preserved (only the epaulettes were removed)

Russian military uniforms are greatly influenced by French military uniforms (just look at the French red breeches that came down in the same strain during World War I), and there are differences in clothing colors and pants colors.

As for why they are blue pants, to be honest, I can't find the source. Please forgive me.