Hi Pollycat, any chance you can post a picture? It's not unusual to lose the bottom most leaves as a phal grows, but the worrying part seems to be the blackness at the base. It could be that when you have watered, some has got into the bit where the leaf joins the stem, if you like, and has caused it to rot. However, a picture would help a great deal in finding out what has happened.
Also, just noticed it was your first post so a big 'Hi', 'Hello' and 'Welcome' to OB