Sean,
If they used pigments in the fiberglass that are similar to those used in coatings (I suspect they would be very similar as the pigments are cheap commodities & would have the same function in fiberglass...color & hiding the substrate) then there may be some explanation as carbon-black is used in grey toned to black pigmenting. White pigment would typically be titanium dioxide...years ago it was "white lead" = lead oxide that is the root of the pre-mid seventies "lead paint" issue.
If they used pigments in the fiberglass that are similar to those used in coatings (I suspect they would be very similar as the pigments are cheap commodities & would have the same function in fiberglass...color & hiding the substrate) then there may be some explanation as carbon-black is used in grey toned to black pigmenting. White pigment would typically be titanium dioxide...years ago it was "white lead" = lead oxide that is the root of the pre-mid seventies "lead paint" issue.