Phalaenopsis: Roots Turned Black after Repotting

[Chitmacha]

While I have many species of orchid that I care for successfully, the phalaenopsis gives me the most trouble when trying to re-pot it.

I am posting pictures of the perfectly healthy roots, which have now turned black, after being mounted; final pic. Although, the leaves still appear healthy for now. But, if I follow past plant behavior, this will slowly changes.

Does anyone have suggestions as to what I'm doing to cause this. (I get the same result whether or not I mount or pot.)

Thanks.