There is not such thing as black magic or curses, the way you know them. You cannot bring the universal forces to do harm to another in any way. Bad luck or accidents as you call them are not a matter of another instructing the universe against you nor a matter of previous life karma. You can into this life to thrive and not suffer. Such matters happen because you expect them to and summon them forth unintentionally. The way the law of attraction works, what you put focus upon, is.
Unfortunately, there are some physical friends that have further themselves from purity, love and brotherhood. As they lay in shadows themselves, they darken their life and do not hesitate to do that for others as well. When it comes to that, they attract negative entities and spirits that have similar intentions. The same as good people and bad, spirits exist of both as well, and are attracted when equal levels of frequency and emotion are arisen. Their negative emotions and intentions towards another may cause a spirit of that sort to attach to the person and drain its energy or cause it negative emotions. Spirits go as they like and work with anyone they like, so that does not mean anyone who attempts a curse as you call it, will succeed, but nevertheless there is an accuracy to the assumption of curse accompanied by a negative result.
In any regard, black magic or curses are nothing else than lowering of a vibration and emotion that then connects to negative spirits. Needless to say, people who play with fire as you say, may as well get burnt. If you consume one with any negative entities of that sort, even in means of hurting another, you may attach it upon yourself as negative spirits are tricky and do not care for rules.
Spells and witchcraft are the same as working with belief that one matter will work but summoning negative energies to attain it. You mustn’t summon or work with any of those unless you are in agreement with attracting and attaching negative energies upon yourself, that may twist your emotions and drain your energy.
Protecting oneself against dark spirits is not difficult at all. As you understand spirits like that are attracted by negative emotions of fear, jealously and so forth, so evidently are repulsed by the opposite emotions of happiness, love, and kindness. When one consumes those stronger emotions nothing negative can attach or linger close to them. When one finds need to call for help, a prayer is a powerful tool for protection against darkness. Always remember that light is so much more powerful and so consume with positive and loving thoughts and do not worry or care for negative spirits affecting you nor negative attention of others as if anyone tries to harm you, they will find it upon themselves.
We are always present in the light.
So much love to all.