Any magic which practices the use of nocturnal energy present around us, for establishing a fulfillment of wish is known as black magic. Nocturnal energies are energies or spirits who are awake and strong during the night time when human beings sleep. It is such believed that at an hour of night the passage between hell and earth opens up and these nocturnal energies are released out from the hell where they reside to the earth.
Since magic has no color of itself, hence people who defined magic and categorized them had bought forth black magic as a category of magic because they believed that those who practiced black magic that is those who took help of nocturnal energies were able to cast more powerful spells, reason being the nocturnal energies would perform their task at an hour of night when other energies and human beings are resting, hence they would not be able to defend themselves from these nocturnal energies.
Wherein on the other hand voodoo is found to be a religion, long time back voodoo was practiced by those living in Haiti , but later with the bringing of slaves to other countries this religion spread and came to be known as voodoo. The practice of voodoo generally combines performing rituals to different forms of deities and it is expected that if the deities are happy with the rituals performed then they will help the person or grant a wish to that person.
Since most of the rituals were performed during night time keeping in mind that the spirit of these deities can be invoked only after sunset or during moon phases hence it got started believing that voodoo uses black magic. In real voodoo does not use black magic but is a religion in itself which involves performing rituals and offer offerings to different form of deities. Even voodoo has special priest and priestess known as the hogans who study the voodoo religion properly and who have the adequate idea about performing voodoo rituals so as to make a deity happy.
Many learned people have debated on this topic of whether voodoo and black magic are the same or not, but if we closely observe and study the aspects of both black magic and voodoo we will realize that voodoo is a religion whereas black magic is a technique. Voodoo uses similar techniques to complete the rituals in its religion hence many do stand in the favor that voodoo and black magic are the same.