What could cause a blue or yellow parakeet's tail feathers to suddenly turn black?

A blue or yellow parakeet's tail feathers turning black could be caused by a few factors, including stress, illness, or nutritional deficiencies. Feather discoloration can also indicate a fungal or bacterial infection, or exposure to toxins. Additionally, feather damage from aggressive behavior or environmental factors may lead to color changes. If this occurs, it's advisable to consult an avian veterinarian for a proper diagnosis and treatment.