It might not be fully dry.
Black ash can be caused by all sorts of things that don't really mean anything negative... though overfeeding can cause it too and weed that is signficantly overfed will also have a hard time staying lit... so will slightly moist weed. If it goes away, you can chalk it up to moisture... if even after curing longer it still is dark and hard to stay lit, it was probably overfed for a long time and caused some odd flower construction.
once a plant is toxic, flushing does not change the plant (directly). flushing is a tool to impact the root zone nutrient content - outside the plant, not inside. This is why flushing for no reason at end of grow cycle is virtually no impact on the plant except if it is starved of useful buidling blocks - even trichomes need to be built from 'something,' Then, it's porbably a net-negative vs irrelevant... irrelevant being the best potential effect, lol.
A plant has no method of excreting excess nutes. adding water won't dilute the plant inside. over time taking in less compared to rate of use will draw it down inside the plant. it's never 'flushed out'... never.