cymbidium leaf turning black

[Cym Ladye]

Suzanne is probably right if the black is coming from the crown of the bulb. If the black 'Notch' is somewhere else, who knows without a photo.

If the whole leaf is black, remove it gently and all the other leaves from the young growth. I suspect you will see a black crown. If you are lucky, this may not have spread beyond the new growth. You can use a sterile razor blade and gently scrape off the black, being careful not to injure the base with a new growth bud. Then Then sprinkle powdered cinnamon on the remaining top of the growth only and leave it open to the air.

This generally occurs when water stands in the crown too long. Happens with phals too and is usually the death of them. In Cyms. it just means the loss of the new growth unless it has started from the base of the bulb and that is a whole different problem.