Have you tried treating it with a systemic fungicide? Do you flush the pot very well with distilled/rain water to avoid fertilizer build up? What do you grow it in? Did you look up the parentage to find how they are grown (some cattleyas like more shade, some need very bright light, some prefer to be mounted, etc.). Do you keep the leaves dry after watering? What sort of calcium product do you use? Do you have adequate light and air circulation? Does your fertilizer contain trace elements? In what media do you grow? Is the blacked part of the leaf dry?
I know how frustrating it can be as I dealt with black rot. This year, many of my orchids are recovering instead of blooming. Good luck!
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