The color black in general strongly absorbs light. Therefore,  black fur attenuates light transmission at least 10 fold. Depending on signal intensity one may need to shave the mouse or use Nair (sensitive skin). If working with Bl/6 mice, you can use the syngeneic albino variant C57BL/6-Tyrc-2J from Jackson Laboratories. White fur does not absorb light.  Of course, one can also opt for a hairless immunocompetent mouse such as the SKH1 mouse from Charles River. For xenografting, the following immunodeficient models are commonly used.

For fluorescence in vivo imaging, it may be imperative to shave, depilate or use hairless mice since different densities of hair create different levels of autofluorescence background.