The black spots could be one of the fungal diseases that infect plumeria especially after a warm, wet spring. You can use a fungicide on the tree or wait until summer weather when these fungi usually clear up. To help stop the spread of disease look for ants or aphid that transfer the disease. You can use Tanglefoot a sticky goo  around the trunk of the tree to stop ants from climbing up and tending the aphid. The honeydew or aphid droppings encourage the fungi to spread. Keep growing, Marianne Binetti