Could well contain carbon.
http://www.specialchem4coatings.com/...book/?id=black




From that site:

Flocculation
Following dispersion small particles of carbon black have a tendency to
flocculate. To minimize this tendency it is necessary to take utmost care
when diluting paints. Only small calculated additions should be made with
thorough homogenization between additions



So it could be that over time, whatever is used as a pigment "flocculates"
and forms solid flakes or a layer of conductivity on the PCB.

(Being as the pigment particles are not dissolved in a medium as a solution,
but are merely mixed with it)



Gareth.