I don't think it's sunburn, mainly based upon the patterning of the blotches ie those concentrated black blobs. I actually had a similar problem one year, when I decided to grow my plants out doors, and lots of my them acquired brown blobs on leaves and pbulbs. The brown patches remained quite dry and hard, but spread slowly. I got the infection under control by treating with phyton 27 (repeatedly). The patches turned completely dry, black, and stopped spreading.

I heard somewhere that bacterial infections typically spread quite quickly, and result in mushy patches due to the rapid necrosis and collapse of the cells, whereas fungal infections are slower and are not as mushy. Not sure if this is a hard set rule though. It seems strange that you've had this for over a year and it's still growing - that's really slow.

If it were my plant, since it looks like you have lots of pseudobulbs, I would cut one off and get it sent off to some university that can figure out whether something needs to be done about it.