you don't get any darker than a minor key. You could experiment around with C# harmonic minor, but no other relative minor modes get as "dark" sounding as the Aeolian (minor) mode.
...Unless you play around with modes other than the relative that is.
With you having limited theory knowledge I won't go into great detail but there are several kinds of minor scales.
Aeolian (natural minor)
Dorian
Phrygian
Harmonic minor
melodic minor
pentatonic minor
to name the most common ones.
without hearing your backing track it is impossible to recommend the correct scale choice, however.
I am a music teacher of 14 years or more and I hold a grade 8 with merit in rock/jazz theory from RGT London.
I say this as I have considered starting a theory thread here with a list of lessons on guitar theory with the chance to have questions answered.
Would that be of interest to many here? Is it worth the amount of time it would take in other words?
There are so many issues with learning theory, especially modes.
I remember I was given a book of scales to learn when I was a teenager. It said it had every scale a guitarist NEEDS to know. Well there were hundreds of patterns to learn in this book with as many pages and it put me off for long time.
I have since come up with my own method for learning modes which is a combination of fretboard knowledge and using theory in real time, a little bit of work but the result is the ability to play almost any mode at the drop of a hat without having to memorize a million different patterns in 3000 different keys lol.
If enough people express interest I will try to get some info down in the next few days. The issue is it does require theory knowledge to work so I would start almost right at the beginning so it would take many mini lessons to get through it all.
Cheers.