It happens regularly when they come in after p&h, even in warm enough season. Maybe it's just the substrate too wet for the few days needed… It can start a few weeks after coming in!
I've still to find a reliable method to avoid this but it doesn't do for leafless species. Leafy ones seem responding better to repot/dry keeping on the very dry side for a month (I'm not saying I don't water).
Anyway, once the black stops, and you see now roots or a growth, it's ok. But this will occur only after resuming normal watering. And NEVER overwater a vanilla not doing anything, the first cold it'll feel will start the thing and here it has nothing to do with temps, it happens in 24-26°C here.
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