Recently planted bird-of-paradise in a large pot in a sunny part of my yard. There are now black blotches on the leaves.
Black Blotches on Leaves of Bird of Paradise
Certified GKH Gardening Expert
The black blotches are most likely sooty mold which means you have an insect infestation (either aphids, scale, mealybugs or whiteflies). I would inspect your plant thoroughly for signs of insects. I have included links to articles below that will assist you in finding the culprit and provide you with a course of treatment: