Lawyered.No, certain bond types (conjugated multiple bonds most often) absorb light at a certain part of the spectrum. What you see is the result of the absorption, the resultant light that is left over is what you see. So for example carotene (a molecule with multiple conjugated double bonds) absorbs in the blue/green area of the visible spectrum and you see orange as the colour "left over" from this absorption. Hence why carrots are orange. Back to why you see things with a black light....it's not dirt, it's just bodily fluids with the same characteristics described above in terms of molecular structure but this time they are absorbing in the UV area of the spectrum rather than the visible.