I am aware that the weather changes often and that I should wear layers, but I also read that locals tend to dress quite modestly (dark, plain clothing). Is that so? I will note that I am going to a few metal concerts and festivals - should I still dress classy?
The clothing locals wear will change with fashion, so what was noticed last year will likely not be the same this year and will be different again next year. And what you describe sounds like office workers on their way to work.
You will be a foreign visitor and as such you will stand out whatever you do, it is almost impossible to blend in and it is useless as it will not help you with anything.
If you go as a tourist, I would dress in the style you would when going to a different part of your own country, when going into town you would wear a milder version of your style than you would when going to a concert. That will do for Switzerland as well.
Being dressed in 'metal' styles will likely help you get in with people who also like that music, while when being dressed as an office worker will not get you anything.
On the other hand, if you plan to combine business with metal concerts in one trip, you best bring business style clothing. You do not need to adjust to what you assume is Swiss style, what you would wear at home going to work or business casual if your contacts allow it.