Blacars are "other standard" extra-planar entities in the I.C.E. Age lore which did not exist as actual creatures in the game. They manifest in material planes of existence as floating black spheres with diameters of approximately 1' to 1.5'. While peaceful in the plane of pure energy they come from, they are driven mad by material worlds and assault those they encounter by feeding on their mental strength. They swoop through intelligent beings and cause cold criticals as they pass through the body. Those who suffer their attacks badly enough will eventually die from having lost the will to live.
There is reason to believe Uthex Kathiasas had Blacars in The Broken Lands. It seems to imply they are related the dark vorteces, or that the vorteces are unnatural Nycorac/Blacar hybrids, which were very similar creatures of pure energy who also fed on the mind of their victims. These are relevant to the apparent mechanism of telepathic control by the crystal dome. They are not demonic.
Behind the Scenes
Blacars would have been an especially unusual creature because their attacks would drain the stat points of characters, like the old death mechanics, eventually taking away all discipline and self-control. The dark vorteces deviate from their Rolemaster lore by being visible and causing cold criticals, which most likely came from the Blacar lore instead. This would be the reason the vorteces drop hazy tenebrous orbs. Blacars would presumably be birthed from the death of vorteces, possibly representing a transitional stage of the same life form. Virtually nothing was known about the vorteces.
These stones probably represent the Blacars as a nest of not-fully-developed-yet vortece eggs. The vorteces have also been observed shrinking and gradually dissipating in intensity when they are in the huge cavern, so from the other direction the orbs can also be viewed as a form of corpse. They are the same color as the hazy tenebrous orbs. While there does not seem to be any obvious parallel in this case to a Lovecraft story, they could correspond to the large eggs of the night-gaunt fearing, Nyarlathotep serving huge shantak birds who flew over plateau of Leng to the monastery. This is important because the shantak resemble the Terrorite demon of Shadow Valley, which emerged from an underground portal, but would originally have been located near the hidden portal to The Broken Lands.
[Dark Grotto, Cavern] The floor of this small round cavern dips suddenly down, forming a kind of bowl with the lowest point at the center of the cavern. Large round stones, all of them roughly the same size and shape, lie scattered around the cavern, but for the most part are heaped in a loose pile at the center of the cavern. Obvious exits: north >look stone The stones are fairly uniform in size, shape and color. They are all dark grey balls of smooth stone, about ten inches in diameter. >take orb You pick up a hazy tenebrous orb. >look orb A dark grey mist appears trapped within the glass-like orb. Despite its lifelike resemblance to a ball of early morning fog, the thick mist remains unmoving, maintaining its perfect nebulous form. Twisted shadows linger within the orb where the ambient light is unable to penetrate the heavy grey haze.
The orbs may also be a reference to the visions of the Shining Trapezohedron from "The Haunter of the Dark", which summoned the avatar of Nyarlathotep and was probably the basis of the crystal dome on the Jagged Plain. These "alien orbs" refer to other worlds "with great stone towers, and other orbs with titan mountains and no mark of life, and still remoter spaces where only a stirring in vague blacknesses told of the presence of consciousness and will." The character saw hooded figures and "vortices of space where wisps of black mist floated before thin shimmerings of cold purple haze."