"Black magic" Is an idea, there is no such thing as magic with a colour. Magic is energy, and if you are not using energy for an evil intent, there is no way it could be evil. Many pagans do not even believe in karma or the three-fold "law", as that is a wiccan idea.
My first piece of advice - Do not mess with free will. You broke up, you will move on. Odds are, if you broke up, it was for a reason.
Most likely, nothing you send him in text could make him feel like that. You could try a get-back-together spell, but you cannot really send a text to send a spell. (even though some say emoji spells are a thing)