To Be, Is My Answer

The reason it was so important for narrative purposes, was that the plot concerns the visit of the Astronaut, in his completely opaque spacesuit, to a planet populated entirely by self-aware robots (originally from Earth) who have built their own society and are petitioning to be allowed to interact with Earth again as equals.

They have a democratic government and free choice of careers etc. as the orange robot serving as guide tells the Astronaut.

The Astronaut notices that there are two different types of robot on this world; the orange ones, who are in charge, gifted access to all information and facilities. and the blue robots, who are seen as more limited in function, have less access to information and resources, and are not allowed positions of power or as wide a choice of employment opportunities. Even transportation is segregated.

The Astronaut investigates further and discovers that the blue and orange robots are actually structurally identical, there is absolutely no difference between their potential or capabilities, and it is only because the orange robots are instructed by their Educator system to consider themselves superior, that the difference exists.

The Astronaut tells the robots they are not ready for re-alignment with Earth, until they come to terms with their own unfairness, and how Earth had had to deal with this issue themselves. When that time comes, the robots will be able to ally with Earth.

Then he leaves in his spaceship, and it's only in that one final panel that we see the Astronaut is black.

Not subtle, nor should it be, but for 1950 this was a breathtakingly powerful statement, perhaps the first of it's kind in the genre.