Throughout my career I have observed athletes disappear into dark holes never to reappear or if they did they were a shadow of their former selves. What is an athletic dark hole? It is biased one sided training that emphasizes development of one physical quality to the exclusion of all others.
The most frequently occurring dark holes are strength training to the exclusion of everything else or an emphasis on aerobic work to build a base that takes away all explosiveness. Neither is bad unto themselves but without context they possess the potential to kill an athlete. All components of training must be trained during all phases of the training year in the context of the objectives of that phase and the needs of the individual athlete.