What does the author mean by the “color line”?

The "Blue Veins", a group established after the Civil War, was mainly comprised of African Americans who were "more white" than black, though they denied this. In other words, the color-line, seems to be a distinction made by the African Americans themselves.... a classification or social hierachy created by African Americans post-war. The color-line applied to a perception of whiteness.... the lightness of the skin, the visibility of blue veins, and the culture and character of its members.

From the text:

Mr. Ryder might aptly be called the dean of the Blue Veins. The original Blue Veins were a little society of colored persons organized in a certain Northern city shortly after the war. Its purpose was to establish and maintain correct social standards among a people whose social condition presented almost unlimited room for improvement. By accident, combined perhaps with some natural affinity, the society consisted of individuals who were, generally speaking, more white than black.

The Blue Veins did not allow that any such requirement existed for admission to their circle, but, on
the contrary, declared that character and culture were the only things considered; and that if most of their
members were light-colored, it was because such persons, as a rule, had had better opportunities to qualify
themselves for membership.

Mrs. Dixon had come to Groveland from Washington in the spring, and before the summer was over she had won Mr. Ryder’s heart. She possessed many attractive qualities. She was much younger than he; in fact, he was old enough to have been her father, though no one knew exactly how old he was. She was whiter than he, and better educated. She had moved in the best colored society of the country, at Washington, and had taught in the schools of that city. Such a superior person had been eagerly welcomed to the Blue Vein Society, and had taken a leading part in its activities.