Recently the pitchers on my Nepenthes pulchra have been dying off. I got the plant a few months ago and it has been growing in the same conditions with 80 degrees in the day and low 60s at night and close to 100% humidity the whole time. I have the plant under CFLs for 10 hours every day. The largest pitcher started turning black a few days ago. It was a few weeks old though, so I didn't think much of it. I had fed it a few ants but that didn't seem to cause a problem.
However yesterday I noticed that the newest pitcher has also begun turning black. I don't even think it has opened yet.
The rest of the foliage doesn't seem to be affected. I have a veitchii and macrophylla under similar conditions but I've only noticed this on the pulchra. The growing medium appears to be a mix of LFS, perlite and peat. I got the plant potted and haven't disturbed it at all. I mist it several times a day but don't water it.
Here is a picture of the whole plant: