Leaf Discolouration

[louise]

Please would you help me, I am quite concerned about my Neofinetia falcata collection which I have grown without any problems. Recently I noticed this deep purple/black discoloration at the base of one plant, which I thought was normal, until it spread over the last few weeks to the underside of the leaves, then the second plant! The leaves of a Dendrobium Peguanum on the same shelf started turning a purple color, I moved that to a shadier area thinking it was too much light, although it has grown in the same place on my light table for 2 years! Nothing else has changed so I am puzzled and worried. Thank you inadvance for your help