First you have to understand that the more sharps a chord has, the brighter it will sound. The more flats, the darker it will sound. But wait, you're thinking, you can just use the end-harmonics to make the sharp flat, so your logic is flawed. Wrong. The nature of an EMajor chord is to be bright, but so is an f flat minor chord. The reasoning is because of how a piano is tuned. If you really want to get into, do some studying on how a piano is tuned. Chords are tuned, not individual notes. That is the most accurate way to do it.
After that lengthy introduction, I'll give you some dark chords.
Try e flat minor or anything added onto it (sus7, 7, 9, 11, 13) Those extras are a nice touch for variety. a flat minor is also fairly dark. If you want to go for dark and tense, try e flat dimminished or a flat dimminished. Remember though, if you create a lot of tension with a dim chord, you're going to want to resolve it by going back to pure minor (or major).