It sounds like a case of camellia flower blight. Plants that have this fungal infection develop flowers that turn brown, especially when there is a lot of moisture present. The entire flower turns brown and then drops in 1-2 days. You can usually see an early symptom in the form of slight browning of the petal edges. Spots on the petals deveop and eventually enlarge to cover the whole petal.

Good sanitation techniques sometimes help. Quickly dispose of any blooms that fall down. Do not leave them out there till the end of the season but patrol the area every day or every few days picking all plant debris that you find. Dispose of plant debris in the trash and not in compost piles. The spores can survive in the soil so you may want to replace the mulch and make sure that water does not accumulate around the plants. You can also apply soil drenches using Southern Ag Terraclor Fungicide 75W in mid winter. Other fungicide sprays recommended for the flowers include mancozeb (Bonide Mancozeb Flowable or Southern Ag Dithane M-45) and triadimefon (Green Light Fung-Away).

Here is a link that might be useful: More info from the A. C. S.

Thanks for the info. In the PNW there is always a lot of spring rain so perhaps this fungal disease is widespread. I don't have a problem with my one camellia that is protected from the rain. The flowers are so lovely I hate to see them turn brown.

Good to know. Use good sanitation techniques when you notice plant debris to keep all fungi away.