how do you even get dark sonic in the first place?

You go onto the 20th alternate skin False Chaos and Rage and get the boost gauge and max to get Dark Super Sonic.

If you use runes, you can just turn on the option for super form and after you get 50 rings or more Down + Taunt will let you go Super sonic for any color, his is Dark sonic meaning you can stay like that till you run out of rings. If you combine that rune with the middle too and spam tricks you can rack up rings quick

chaos emeralds

i think i know what you are talking about try using two players with chaos emeralds buddy equipped then transform, if you don't transform twice you will get the normal form so remeber to get only 7 not 14.
edit: the music gliches so i like when one person has the normal and the other has a non-music buddy