Cattleya percivaliana with black pseudobulb

[estación seca]

Those temperatures are OK for Cattleyas for now. The black pseudobulb may be sunburned. That's what sunburn looks like on Catts.

I don't treat with fungicide unless I'm sure I have a fungus.

[SouthPark]

Does the black coloured region appear on the other side of the bulb too? If sunburn is a candidate, then I would also assume some leaves would get at least some scorching too. Still a possibility though. Check the other side of the bulb too.