There are different versions of "black figures" that are out there...
Typically black figures are shadow people... They can harness enough latent energy to take on a physical form, but cannot become defined enough to manifest themselves as the person they once were. These are the most common causes of hauntings in houses. You see them move out the corner of your eye, or swear something is in the room with you.
Black figures can also be guardians or watchdogs. Bachelor's Grove has at least one such figure, and there are multiple stories spread out over the internet regarding how the figure (usually described as a "he") will chase ne'er-doers from the Grove to protect the area.
Finally, there are those who are essentially demons or malicious spirits intent on causing duress, whether physical or mental, or perhaps even causing someone to die either directly or indirectly. These will usually take on either a physically imposing form (very tall, animal characteristics such as horns, hooves, tails, etc) or a familiar form with a quirk (like the black-eyed people recently discussed). These are the ones that are the trouble-makers, and give bad reputations to most things paranormal. If you even think you might be dealing with something like that, it's time to seek protection.
Most Guardians and Shadow People are harmless. They're keeping an eye on things, and try not to draw attention to themselves unless they are forced to.