Midnight Blue is also quite a common evening wear colour, since that tends to actually appear darker than black does many times. However, since there's often a lot of black in many tartans, it might look odd paired with midnight blue.

Keep in mind that if you're looking for evening wear, there really are rules you should follow. If you'll be one of the few people wearing the kilt to an event, I think black or midnight blue would be best. After all, you wouldn't expect someone to appear at a black tie event wearing a red tuxedo.