Black Magic

There are three factors that determine whether a power is Black Magic or not.

•Source - If it's gained from the Dark Side or a Deal with the Devil, Religion of Evil, or Sealed Evil in a Can, it's almost always Black Magic. Other sources can include channeling hatred, "evil", The Corruption or hellfire. This sometimes but not always includes Elemental Darkness.
•Cost - What is required to make the magic work. Even if it saves kittens, if fueling mana requires stealing others' Life Energy, blood or Human Sacrifice, or even working with dead bodies (because it shows a lack of the respect Due to the Dead), it's probably Black Magic. (Note, using your own Life Energy doesn't necessarily fall under this.) Using Eye of Newt that you had to do horrible things just to collect is also a bad sign.
•Effect - The magic is directly constructed for the suffering, control, and/or needless death of others.

Also beware of the side-effects. Gaining and using Black Magic
will often have a "corrupting" effect on its practitioner's character, causing grey actions to go to black very quickly. This generally coincides with a similar degradation of the practitioner
's mental stability.

Necromancy

Necromancers are the masters of the art of death. Necromancy,
the magic of interrogating the dead, has been practiced since at least Ancient Greece,
where it was known as Necyomantia, and probably earlier.

Necromancy includes piloting the dead. This is a process where a skilled necromancer can control any weak minded individual. Usually this is done with vampires or shifters gone that have lost their mind to blood lust.

If a piloted individual is killed the image of their user can be traced unless the user created an effect to block it. The piloter also has to back out of the individual or they lose their mind. They act like they are dead and lose all sense of self.

Blood magic

Spilling of blood is a potent force in the working of magic. It may be a token sacrifice, but it may also be the loss of life that fuels the spell. Expect mages who practice Blood Magic to be portrayed as evil, or at least dark grey.

Some blood may be indicated to be more powerful than others. Common types are human blood, the blood of royalty, the blood of a special line, a child's blood, or virgin's blood. Sometimes only a single person's blood has power, and any other blood is powerless.

The other side of the coin is menstrual blood, which is used much less frequently but is associated with life magic when it is used. Except, of course, when it's the Menstrual Menace.

This trope is Older Than Feudalism, with blood and sacrifice being powerful magic in some of the oldest tales. A very old Greek curse was for a witch to run three times around the house she wished to curse while menstruating. Especially potent if it was her first period. In more recent works it is frequently the alternative to Necromancy as the "evil magic".

Frequently a part of any ritualistic Blood Bath. When it's the blood itself doing the damage, you have Bloody Murder. When the magic is used to extend the user's lifespan, it's Life Drinker.

which is often written in man's blood on parchment made of human flesh. Is often a cause for a Superhuman Transfusion.