Black bark on coral bark Japanese maple
Bark turned black from pink and tree died within 6 months.
Certified GKH Gardening Expert
This may be a disease, which is difficult to diagnose. You may want to discuss the matter with a certified arborist, or with your local Cooperative Extension Office. (https://www.gardeningknowhow.com/extension-search/)
This information about Japanese maple diseases may help: https://www.gardeningknowhow.com/ornamental/trees/maple/maple-tree-bark-disease.htm