@msolga,
I'm patting myself on the back for getting my irises and spring bulbs in just ahead of some welcome rainfall; the soil needed a good soak, it was so dry. This darkness is ,like you say, a normal part of seasonal change, being located under the cold air-warm air boundary.
Just last week I came across this out of season lilac blossom:
I marked that stem to see if it blooms this time next year to determine if I have a significant mutation on that plant.