The is still much unknown about the capabilities of black slime, even the Gozerian cult had only scratched the surface of its potential. It is however known to be extremely toxic to body, mind, and soul. The fact of Black Slime entities (at least the larger ones) seem to manifest in a humanoid form does raise some questions. One theory, based on unusual encasing devices found on Shandor island is that some people, weather prisoners or willing cult members is unknown, are locked into the device, which is then filled with black slime. This toxic substance consumes the victim both body and soul. As such, the human shape maybe due to residual will from these sacrificed people.
The pages in Shandors book in regards to black slime give great detail about the luring and capture of the infant Sloar and harvesting of the black slime. It also gives a bit of detail about the Sloars home dimension. It is not recommended that this be researched by anyone unfamiliar with the concepts and nuances of cosmic horror.