Sooty growth often comes after aphid or mealybugs infestations. It is sooty mold and developes on the juice from the plant.
In the joint where a Vanda leaf meets the stem, it often gets dirty. No cause for worry or concern. Sooty mold typically is not found on Vandas.
If the black marks are more like dashes - —, that is a fungal problem. It needs immediate attention with a systemic fungicide like Thyomil. It can't be cured by "home remedies", it is serious and could weaken the plant. A strong disease requires serious treatment.
Vandas like lots of sun, lots of water and lots of fertilizer to reach their full potential.
