what are the blue veins. what are their valves. what role do they believe they play in the contrasting social class of white and blacks?

The story opens with Mr. Ryder, a light-skinned black man born free before the Civil War, planning a society ball. He is the head of the Blue Veins Society, a club for educated and cultured black people who all, interestingly enough, tend to have very light skin (hence their name, for with light enough skin their blue veins are visible beneath).

What roles does blue viens play in contrasting social class of blacks and whites