My phaius orchids new growth keep developing rot.

[doctorw]

I bought a phaius orchid a few months back and recently I moved it into a slightly large pot as it was root bound. Just before I repotted it I noticed some of the new growths starting to come out of the pseudo bulb had blackened and after I repotted it more growths have blackened as well as a bit of the pseudo bulb itself. What should I do? Its seems to start as a blue grey spot and spread from there. Also Ive only watered it when the substrates gone dry.