Black spots / patches on underside of cattleya leaves

[mexicowpants]

Hi, bought a young cattleya "amatista" a few months back at an orchid show, during this time it hasn't seemed to grow very much at all, despite the same care as all my other catts, which are thriving.
Today I noticed that the underside of 2 of the leaves are more than half-covered in black patches / spots, which feel a little softer than the surrounding leaf tissue, and slightly sunken. Another couple of leaves are affected to a much lesser extent. I only just noticed it today because it's quite a small plant and only the underside of the leaves are affected.

My guess is a fungal or bacterial infection, although I'm unsure which. I've treated it with an alcohol rub so far, but was hoping one of the more knowledgeable people here could offer up some advice.

Thanks in advance!