Black substance on circuit boards?

We coat all our circuits with a conformal coating to protect the leads and contacts to protect them from environmental issues. At the speeds the circuits are running, any two neighboring leads act as capacitors. If a piece of dust or a drop of moisture lands on them they might as well be a short circuit even though they are not touching. If that happens, at best, you have incorrect results, at worst your circuit is destroyed.
The only way we can assure a known dielectric between the leads is to coat them with something that has well defined and constant properties over the entire temperature and humidity range. The added step costs money but it is well worth it for the manufacturer and the customer.

What is the conformal coating? some sort of resin or wax? the problem with these is that sometimes, the product overheats and those coatings melt causing the product to short circuit anyway. What are they made of?

It could be a chip on board (CoB) construction. A bare chip is mounted directly on to the pcb and wired directly in to the circuit board. The thing is finished off with a blob epoxy to hold everything in place and protect the chip.

It’s a much cheaper production method than encasing the chip it to a package and then fixing that to pcb (or via a socket like for example desktop CPUs).

@cazzie Each one is different depending upon the environment, circuit speeds, substrate and the spacing, the coating can be a UV cured epoxy, or a ceramic two part or even the old standard Sylgard that has been around for decades.