The purplish coloration of the leaf leads me to believe that it's a fungal infection such as black rot fungus, which starts out purplish in color, turns black and eventually causes soft rotted areas. If left unchecked it can spread to the rhizomes and the roots.
My second suspicion is a virus, but I'm leaning toward my first and if you googled cattleya black rot fungus, you will see images similar to yours.
If that is what it is, you can drench the orchid in fungicide preferably with something that has etridiazole (sp?) "Terrazole", which is kind of expensive.
Again, this is Guess Work on my part. Waiting to hear from others. Good Luck
I just googled it and ran into a good article by AOS, may want to read it.