Camellia flowers turning brown are a common problem. Some of the factors that damage the flowers and cause them to turn brown and drop are frost, wind and prolonged wet. Unfortunately, these conditions are common in our area during this part of the year. Deadheading, and removal of flowers helps the shrub look better. This article has additional information, Camellia Flowers turning Brown and falling off https://internationalcamellia.org/camellia-faq.

There is also an infectious disease that can cause browning on camellia flowers, called Camellia petal blight. It’s an infectious disease caused by the fungus, Ciborinia camelliae. With this disease you should see small irregular spots that grow to cover most of the flower. The development of this disease is favored by wet, humid conditions and mild tempertures (59-70° F). It’s important to remove affected flowers and petals, and to clean up fallen leaves and flowers because they contain the spores that will reinfect the camellia. This refuse should be bagged and tossed with the regular garbage This article has additional information, Camellia – Petal and Flower Blight https://pnwhandbooks.org/plantdisease/host-disease/camellia-flower-petal-blight.