The terms black magic and white magic are man made ideas, that actually have racist undertones. The idea was if you were a lightly skinned person you were "good" and thus used "white magic". Though if you were dark skinned , like a lot of people from eastern practices , such as hoodoo and others. In reality , white and black magic are terms to generally refer to workings that produce "good" or "bad" effects. However what's considered good and bad depend on your perspective. Say, I kill a murderer, am I a good person because I saved a potential victim or a bad person because I took a life? In practices now a days, the terms white and black magic and changed and referred to as something like "positive" or "negative" workings. So, "black magic" isn't harmful, its the concept of magic used to cause a negative situation or negative effect within another person's life. Now, wither or not that counts as harming is up to the witch. Because say I cast a spell to curse someone because they leave me no choice, my intention would be that I want this person to stop doing whatever they're doing that is hurting me and I'm stopping them through the act of causing harm to them. Now because my end goal isn't "hurt them for no reason" my working is not considered causing harm from my own point of view. Hope that makes sense to you.
I responded to your question in the articles, but the short answer is, it's a scare tactic. Magick is intent, not colour [as said above and in my article response, it's based on racist constructs] If you understand how the energy works, you can cast any spell. As for the harm, negative energy lingers longer than positive energy. People don't tend to cleanse properly after casting a baneful spell and the negative energy they charged starts affecting their life negatively. If you clean up after your spell, you shouldn't have any problem [I've cast a number of baneful spells with no negative repercussions. practice cleansing, shielding, warding and/or protection spells and you should be fine]
As Nekosaid, energy after casting if not directed properly can hang around. Having proper protections in place can help.
Another consideration is whom you are casting against. Non practitioners can still have protections from ancestors and other spirits fond of them. Some without knowing it.
Casting against another whom has protections in place can backfire.
These are examples of how negative workings can potentially be harmful for the caster. Proper protection and common sense (knowing when it isn't in one's best interest to cast) can help in some circumstances to lessen potential backfires.
Different traditions define or describe things their own ways. Of ones I know off-hand which use back versus white magic, there are three leading general concepts.
In some, white magic is beneficial, such as healing, prosperity, luck, and happiness. In these, black magic is often baneful or harmful, directly relating to curses, hexes, and the like, intending to cause pain or harm to another.
In some, white magic is restricted from causing any harm whatsoever, and this means that things like love spells, call-me and think-of-me spells, and on down that list, are considered black magic for interfering with free will -- seen as harmful by people who hold these views.
In some traditions, black magic is aything for personal gain whatsoever. If your pursuit is external or material, it is considered black magic by some of these. The same will define white magic solely as workings to -- basically -- be the best "you" that you can. White magic is anything improving the internal world, conquering one's own demons, mastery of the self. In some of these traditions, they will insist that self-mastery leads to conditions which mean not only are physical needs typically met because the magician will pursue their best life, but that prosperity, and the desired material things will come to be, again, because the inner world is prosperous and therefore the outer world becomes so.
Black Magic is any energy manipulation that carries with it a negative intention, manifestation, or desire. A love spell can be either white or black depending on what it is that the practitioner is casting for. To increase love and bring a couple closer together a honey jar can be constructed to sweeten dispositions and facilitate harmony in the relationship. However trying to break up a couple for your own benefit is straight up dark magic. Spells for personal gain can be seen as black as well if it's greedy but spells to increase your protection, manifest prosperity, and for healing even though they are for personal gain they are not for greed but one's own survival and are not black magic. It really is based on opinion as to what is considered to fall in which category but most consider revenge, death, or to harm to be black magic. Hexes also are not always black or negative magic. Many hexes are for positive manifestations and can end in beneficial or baneful results. This is why caution should bepracticed as you learn and begin to use energy manipulation. Below are reading materials you may find helpful;
Black magic doesn't necessarily have to equate to harmful, although the term in this context does have a lot of racist and anti-Semitic undertones, as it is usually used to describe practices that are inherently bad or negative. For example, Voodoo and Hoodoo is often viewed as evil or negative due to it's affiliation with traditional African and/or Indigenous spiritual practices (usually seen as idol worship, sorcery, witchcraft, mediumship, etc.), despite many of those practices having assimilated to use the Bible and other aspects from Christianity to survive. Jews in the middle ages were often seen as sorcerers and heretics for practicing the many rituals and studying permitted magic and mysticism of Judaism, often being blamed for the spread of the Plague, missing children, and other shortcomings of the people in the area at the time. The same can be said for the Roma/Romani/Sinti, and other persecuted ethnic groups. Conversely, those who practice Christian witchcraft or work with light, "high vibration" beings like angels are usually seen as good, white magic. Even though these practices and beliefs have just as much potential for harm as the former practices mentioned.
A better word to use when talking about the darker arts is Baneful Magic. This typically includes practices that impact others free will, are meant to bring great harm or destruction, invoking or evoking a spiritual entity for selfish or destructive purposes, or to restrict or eliminate the actions of others. This can be seen as evil or negative, but remember, this is a matter of perspective. Another person may view these practices as just and deserved. These practices can not only cause harm to distress to others, but may also lead to the development of one's own identity and lead them further along their personal journey as a practitioner.
Whatever their reasons are, it's no one's business but their own. And just as all humans are fallible, who are we to judge another practitioner for the methods they choose to get what they need done?