if that black area is soft, you've got some sort of crown or bacterial
rot.

1) isolate plant asap

2) pull it out of the pot, clean off all the old medium, treat the
black area with hydrogen peroxide or one of the physans (or even
remove the leaf and cut away as much of the black as you can reach)
and then dust with cinnamon

3) repot and keep pouring H2O2 or physan on it about once a week.

see also:

http://www.firstrays.com/remedies.htm

under 'fungicides' and 'crown rot'

and

http://orchidexchange.com/Photos/stemrot.htm



if however the black area is hard and dry, i'd just rinse it down with
H2O2 when you repot and keep an eye on it.

--j_a