This made me wonder: which is worse an illegal scam or a legal one? No one would condone overcharging the elderly for this work, but was she better off with the black paint instead of a coal tar pavement sealer?
The facts say "yes;" here's why.
From a cosmetic perspective, the black paint may look good for a little while, just like a coal tar sealer and it would provide some moisture resistance, and it won't have any of the following negative consequences.
From an environmental perspective, the air and water coming in contact will be cleaner if comparing black paint with a coal tar sealer.
From a human health perspective, the toxicity of dried black paint is only a problem if it contains lead, which was taken off the market years ago. On the other hand, coal tar pavement sealers are a potent source of polycyclic aromatic hydrocarbons or PAHs. According to the EPA, PAHs are of environmental concern because several are toxic, carcinogenic, mutagenic, and/or teratogenic (causing birth defects) to aquatic life, and seven are probable human carcinogens.
From a real estate value perspective, the black paint will not have the lingering uncertainty that if hazardous waste cleanup standards were applied, then it might trigger an expensive cleanup. Coal tar sealer has caused site cleanups and soil removal when properties have been redeveloped. Ordinary black paint won't cause that to happen.
So which would you choose?
Yes there was a time when legitimate businesses were using coal tar sealers on asphalt pavement. No one knew (but should have suspected) the effects of coal tar sealers on humans and the environment.
Now we know differently.