phal roots with dark spots

[msparling]

Hello! As I suspect many find themselves first joining this forum, I have an issue with a couple of my orchids.

I have three NoID phals, which despite having very healthy blooms and leaves, seem to have dark spots on the surface of their roots that are exposed to air.

At first I suspected root-rot, tho none of the roots on the surface were mushy, they were quite firm. One of the phals recently finished blooming, so I decided to repot it to check out the roots, and they were all nice and healthy, there's no dark spots on the roots that were 'buried', and they were all firm and pale green/yellow. These spots seem to only appear on roots exposed to the air.

They almost look like burn marks, so I wondered if perhaps they're getting too much sun. They near a west-facing window that's mostly shaded, they don't get a lot of direct sunlight but they do get some. I've never noticed any pests of any kind, on the phals or any of my other orchids.

They're watered once a week, but I check to see that they're mostly dry before doing so. I fed them once in september and october, with 1/4 strength schultz "bloom booster" orchid fertilizer (high potash, I believe), but I have held off as I was noticing this issue in the beginning of november. I'm rambling. I've got a couple ugly flash-photos, hopefully that will be useful (apologies in advance for the hugeness, using my own hosting site):





I just watered them today, hence the damp appearance. They are otherwise extremely healthy with no problems, so this is kind of a mystery. Thanks in advance