This...
 

What I really need is a miltoniopsis whisperer. My very mature plant has not been happy since outgrowing its previous pot and being transferred to coarse bark. Anyway, I see this on a few of the bulbs. I want to think it's fertilizer burn vs overfertilization (calcifications?)?? Then again, I always want to think that. I mix a 20-10-20 power at 1/4 teaspoon per gallon of water 2x per month (1/4 recommended stength). Otherwise I water weekly- 6 inch pot. Prob overkill? Why must I love milties so much?

Is it just old growth getting this?

Im not certain if they're old growths, they tend to be on the periphery

Are you referring to the black areas? Are they firm? I've sometimes gotten similar on some orchids, and it's not spread, nor seemed to harm the plants (tho I can't say for certain yours is the same) I don't know the cause - mineral deficiency, perhaps?
Does your fertilizer contain minerals? Do you water with tap water, or pure?

Do you flush the media well once a month?

I hope some other members will be chiming in to better help you ...