How to tell monocrystalline from polycrystalline solar panels
You can't trust the "blackness" which is determined by the cells BBAR (Broad Band Anti-Reflective) coatings.  If both cells had ideal coatings there would be no light reflected back and therefore you couldn't tell from this if it was Mono or Poly.  However, no AR coating is ideal, and in fact are far from it, so the fact that you have multiple different crystal orientations with be very evident.  Mono crystalline cells will be very boringly uniform, you might see broader more smooth transitions in a mono crystalline cell but that will be from surface layer (like SiO2) non-uniformity. 100's of nm thickness differences are noticeable by the human eye.  Poly will have abrupt transitions in the patterns.

On Edit:  It is possible to have what are known as Slips and dislocations in a mono-crystaline wafer that would show up as a "grain-like" boundary.  It won't affect  performance of the Si as such and won't be as mottled as Poly-Silicon but my statement above as to uniformity isn't an absolute.