An "opaque surface" refers to an object or material that does not allow light to pass through it. It is not transparent or translucent, so you cannot see through it. Full definition
Light slows all the time: photons passing through water decelerate to roughly 224,844,344 meters per second, and they stop and are obliterated when they hit opaque surfaces. (scientificamerican.com)
If, then, in its broad definition, abstraction serves to reveal the unseen, for both DeFeo and Grotjahn, it perversely serves as a material diary of intense meditations whose specific revelations remain as impenetrable as their characteristically opaque surface marks. (hammer.ucla.edu)
Silke Otto - Knapp (b. 1970, Germany): Silke Otto - Knapp's paintings are characterised by graphic lines and opaque surfaces with staged representations of figures and landscapes. (museumpublicity.com)