Over the past couple of years one of my Christmas Cacti has developed dark green segments with purplish-red center veins as shown below. I plan to repot it this spring.

The other CCs I have, including cuttings previously taken from the mother plant and potted in the same mix and fertilized the same don't have the dark leaves or veins as shown above the blooms in the picture below.

Are dark veins an indication of some kind of problem or normal if the plant gets quite a bit of light? The plant is kept in a corner with windows on both walls near the plant. I don't see any bugs on it or signs of rot and the plant came into full bloom this year. My late mother-in law gave me the plant so I want to keep it healthy. It's close to 70 years old!