If you have a dark chord progression and want to make it a bit darker, here's two tips to do so.
The Base Chord Progression
Let's take this chord progression.
Here's what it sounds like:
And here's what it looks like:

Let's jump into the tips.
Thicken Up The Last Set Of Chords
The last set of chords, I'm just going to add another note. This gives a nice contrast to the previous three chords.
Here's what it sounds like
And here's what it looks like:

Play One Note Out Of Key
Again we'll do this for the last chords in the progression. We'll take one note and play it out of key. We'll combine it with the first tip as well.
Here's what it sounds like
And here's what it looks like:

Experiment
As always, experiment and have fun with trying different sounds. Find what works for you.
If you have a simple dark chord progression, there's two tips to make them sounds a bit more darker.
This is great for adding contrast and variety.