Black magic -- Magie Noire
Modern magical woman -- Magicienne moderne
Take control of your power. -- Contrôle ton/votre pouvoir. (depends on who you're talking to. Use "ton" if whoever is talking is older/knows the character very well/is on first-name basis with them, etc. Use "votre" if they don't know each other or if they have too much respect for each other to be on first-name basis.)
Everyone has a dark side. -- On a tous notre côté obscur.
Use it to your advantage. -- Tire-s-en profit.