hey guys, im currently making a riff of a song i made, im trying to get this sad sound, so i was thinking what would be a good scale to apply to my riff..my chords are bm-g-d-a..
B natural minor should work fine. I would maybe think about using major 7th chords on the D and A too. Major 7ths sound a bit more complex than major triads so they can help make a piece sound a little darker
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It doesn't matter what scale you use, it's how you use it.

People usually say that a song in a major key sounds happy and a song in a minor key sounds sad. That's usually true, but you could write a song in a major key and make it sound sad if you wanted to...

Anyways, B minor would be a good scale to use
Personally I tend to play more melancholic in A Major and B Major than in any other scale. Those two and D Minor are my preferred scales for playing sad stuff.

But yeah, as the other two said, scales are pretty redundant if you know how to work them.
stay away from the harmonic minor. it tends to take the roll of the bad-ass minor.