PCB opaque coating for anti reverse engineering?
There is no such thing. If someone really wants to reverse engineer your circuit board, they will be able to do it. The only question is how much trouble, and therefore expense and time, it will take.
At best, you can make it too difficult for the casual copier, although that's probably not who you are worried about. The closest thing that matches your spec is to "pot" the circuit board. There is stuff called potting compound specifically for this purpose. There are many different types, from 2-part epoxy mixes, to goop that cures over time or with heat. Each has their own set of hassles and expense at your end.
If you do still end up doing this, make sure to use material specifically intended for this. Some of the potting compounds are silicone based, but there is a wide range of silicones. Some emit acetic acid as a byproduct of curing, for example. Those won't be sold as potting compound for electronic circuits. But someone seeing silocone potting compound, and then the acid-cure stuff cheaper may have a bright idea how to save money.
Silicone is usually transparent. Butalene rubber is sometimes used for potting, especially for high voltage circuits. It's really sticky and gooey stuff until cured, yucc.
Before you go potting your circuit board, think carefully about whether the advantages are really worth the significant cost on your end. Potting won't slow down much a determined cloner that already has the equipment in place. It that's who is going to copy your circuit, you are actually doing the cloner a favor by making your product more expensive than it needs to be and allowing him a nice margin to undercut you. Potting also has other drawbacks beyond just the expense. It makes the product heavier, allows for less power dissipation of components, can trap unwanted dirt and moisture, and makes diagnosing of field failures difficult.
In summary, don't pot to prevent cloning, since it won't. Pot if you need a high voltage barrier, want to withstand a harsh environment, or want to add mechanical ruggedness.
I think the goal is silly, but the obvious answer is to make a multi-layer board and hide all the traces on the inner layers.