Be Preapred for the ingenuity of your PCs!
As stated, Deeper Darkness may be the way to go, or atleast add it aswell to hinder the PCs ability to thwart your well thought out plans. I know this has already been stated, but there is more to consider! (I'm new here so I can't yet comment or I would have done it there, but I think these points are valid enough to warrant a post)
Another way to avoid Darkvision that wouldn't require such a stringent setup is to have your enemies wear a Ring that negates your enemies Darkvision (it's been awhile since I've used it so i can't remember the name of the ring, but it makes hide checks work vs darkvision as well)
However, regardless of how you go about this, Mundane or Magical you have to be prepared for the ingenuity of your PCs.
For instance, without any magical effects (ie, simply a dark room setup as you described), the 1st thing I would likely do as a PC is cast some version of light. (light/dancing light/daylight etc) If for whatever reason that doesn't work, Glitterdust is a fantastic way to light your enemies and your allies (although you may risk blinding your allies as well).
Another trick I have personally used is Detect Magic! True, it only works while concentrating (and on creatures possessing magical items) so I'll have to deal with that if I want to attack, but it will be worth the 0-lvl spell! If you allow talking as a "free/immidiate action" I could also use it to warn my allies: "Fighter watch your left!" (hopefully granting him atleast some of his dex back)
I don't know the level you're planning on using for this, but as long as your enemies don't plan on using magic, (including magical darkness) an anti-magic field would eliminate over 90% of what the PCs can do! Otherwise, you're enemies may only have an advantage for 1-2 rnds.
Lastly, if I'm in that scenario and none of my magic is revealing the enemies, I may just set the carpet/drapes on fire! Sure it may not be the safest thing to do, but it beats getting sneak attacked to death!
I'm sure there are several more ways for the PCs to deal with it. BTW, if they do figure it out, while you may be disapointed that the fight was easier than you intended you may want to consider rewarding the player who figured it out with a little bit of bonus XP. (depending on how long it took and the like...)