Hi, Labradorite is very prone to fracture along cleavage planes. This
can make cutting quite frustrating. Depending upon the raw material,
three out of five stones may develop cracks and become junk. But the
stone doesn’t oxidize or turn colours or do anything fancy like that.
Chances are what made the crack go black is simply oxidation, over
time, of dirt/contamination in any of a multitude of forms, most
likely liquid: a drop of something absorbed from the workbench, oil
from the skin, dishwater, who-knows-what.
To “fix” such a stone is next to impossible and likely not worth the
effort. You might try soaking it in acetone, and if that doesn’t
work, other solvents. Then use opticon crack filler under a vacuum.
I’d say your best bet, though, is to simply soak the stone in hot
paraffin wax for an hour or two.