The best examples of dark magic are the spells that require a living thing’s blood. Also, the magic used to hurt someone is dark magic. Those include dark magic revenge spells, dark magic divorce spells, dark magic stop pregnancy spells, etc.
The spell caster does not physically put darkness into the spell to make it dark or black. The dark magic darkness all depends on the intentions of the casting spell to hurt someone. I am saying this because sometimes good spells may cause problems but these are very rare cases.
Good spells cause problems because of the mistake during spell chanting made by an unprofessional spell caster. Another reason can be the desire already have protection and the spell caster did not check before cast spell and that spell backfire.