I added hamata to my list, but did not add tentaculata because there is no terminology that distinguishes the dark forms from those that produce fairly light colored pitchers. Many species have instances of individual plants which have fairly dark pitchers, without this being the norm.
There's no point in being picky about color "terminology" because there are no valid
Nepenthes varieties or forms described solely based on color. Even among species that you've listed,
N. leonardoi, while noteworthy for sometimes having black upper pitchers, is not commonly very dark either - most uppers are actually yellowish-green. Some others, like
N. ramispina, also lose practically all their color when they make fully mature uppers.
That being said, some less well-known species can get pretty dark;
N. adnata,
N. alba, the true
N. gracillima,
N. muluensis, and
N. rigidifolia come to mind.
<a data-flickr-embed="true" href="https://www.flickr.com/photos/61904224@N05/25990723925/in/album-72157668620943886/" title="N. bongso"><img src="https://c1.staticflickr.com/2/1665/25990723925_aaa707dee7_z.jpg" width="427" height="640" alt="N. bongso"></a><script async src="//embedr.flickr.com/assets/client-code.js" charset="utf-8"></script><a data-flickr-embed="true" href="https://www.flickr.com/photos/61904224@N05/20741401258/in/album-72157668620943886/" title="N. bongso"><img src="https://c1.staticflickr.com/1/732/20741401258_14e193aa60_z.jpg" width="361" height="640" alt="N. bongso"></a><script async src="//embedr.flickr.com/assets/client-code.js" charset="utf-8"></script>
N. bongso - the colors on the second picture are truer to life.
<a data-flickr-embed="true" href="https://www.flickr.com/photos/61904224@N05/14514645159/in/album-72157666216172043/" title="N. tentaculata Murud"><img src="https://c1.staticflickr.com/4/3845/14514645159_1187061dde_z.jpg" width="427" height="640" alt="N. tentaculata Murud"></a><script async src="//embedr.flickr.com/assets/client-code.js" charset="utf-8"></script>
N. tentaculata from G. Murud - has not been pitchering well for me recently, unfortunately.
<a data-flickr-embed="true" href="https://www.flickr.com/photos/61904224@N05/30941674815/in/album-72157666216027703/" title="Nepenthes nigra"><img src="https://c1.staticflickr.com/6/5647/30941674815_447b06cbcf_z.jpg" width="640" height="427" alt="Nepenthes nigra"></a><script async src="//embedr.flickr.com/assets/client-code.js" charset="utf-8"></script>
N. nigra, but it's been more plum than dusky.
<a data-flickr-embed="true" href="https://www.flickr.com/photos/61904224@N05/25311514849/in/album-72157668713881525/" title="Nepenthes graciliflora"><img src="https://c1.staticflickr.com/6/5460/25311514849_3428df2bb0_z.jpg" width="427" height="640" alt="Nepenthes graciliflora"></a><script async src="//embedr.flickr.com/assets/client-code.js" charset="utf-8"></script>
Even
N. graciliflora gets pretty dark under the right conditions.
