If a Compact ACOG's light source fails, does
the reticle revert to black, or cna it not
be seen?

thanks
-z

Yes, the reticle turns black.

Considering the primary light source is the sun - if that fails you've got bigger issues to worry about ;)

If the night illumination (the tritium lamp) fails, then I don't think it will matter much (kinda like using a standard scope at night).

The compact ACOG, as with the other models, uses sunlight as the primary source, and tritium as the secondary source for lighting the reticle.  You can see for yourself by taping over the fiberoptic rod with a piece of electrical tape.  We do this trick to vary the amount of reticle intensity when engaging distance targets in full daylight.  When completely blocked with tape, the reticle turned black. The black reticle (cross hair or donut ring) is much more useful in certain condition.  I don't believe the Tritium would just go kaput without warning since it is a decaying radioactive element.