State and county officials said the stuff is paraffin wax and poses no risk, but they won't venture a guess as to how it got to the Waterfront South neighborhood.
Paraffin wax is used in industrial processes and for making candles.
Camden County Health Department spokeswoman Lorraine Hynes said hot weather probably made the goo more noticeable, melting dabs of it into dark, waxy, half-dollar-sized splotches in an area with industrial facilities and homes.
Solving the mystery became a near-obsession for Bonnie Sanders and other neighborhood residents who feared the stuff was harmful. Sanders, who has lived in Camden her whole life, said she thinks she still doesn't know the truth.
"I believe they're covering it up, I really do," she said.
Some residents still fear for their health and worry the blobs might signal the end of their neighborhood.
When Sanders gives a tour of the area, which is more depressed than most in one of the country's poorest cities, she just points down.
The spots morph over a few days, Sanders explains. The fresh ones look like small oil spills, most of them round, most about 6 inches across. Though they look like liquid, they don't feel like it.
As they dry, the blobs get smaller, darker and look waxy. They end up about the size of a half-dollar and they're not easy to get off the sidewalks.
Some neighbors tell Sanders she's paranoid.
Waterfront South is a neighborhood where factories and homes make uncomfortable neighbors. It is strewn with bits of broken bottles, potato chip wrappers and a few stray pieces of furniture.
While sidewalks everywhere have splotches, the concentration in the neighborhood is in the high-dozens in most sections of concrete.
Sanders fears the stuff is causing health maladies. "A lot of people are complaining about headaches," she said. "New people are coming down with asthma."
Wanda Johnson is skeptical. The 46-year-old salon worker is less concerned about black goo than she is about whether there are secret plans to raze all the homes in the neighborhood.
She squatted down Wednesday and dug into one of the sidewalk blobs with her car key.
"This is black tar off a roof," she declared as she stared at the speck of stuff at the end of her key.
Maybe. Or maybe not.
State and county inspectors visited the neighborhood site last week.
A quick field test wasn't able to confirm that the substance was petroleum-based, said Department of Environmental Protection spokesman Fred Mumford. That seemed to rule out tar from roofs or street repairs and leaky cars.
Officials at more than a dozen industrial facilities and two federal Superfund contaminant sites said they haven't had any operational problems that would cause the splotches, Mumford said.
Bob Lentine, assistant commissioner of the county health department, said it's probably nothing to worry about in any event.
Results of more tests were expected to take a few more days.