Just wiped the inside of my glass with my finger, collected some of the specks, gently ran my other finger over them and they streaked black. WTF?
Hazarding a guess:
If the problem isn't filtration, those flecks could (could) be precipitated salts. If you don't use properly deionized water to reduce the distillate to bottling proof, little flecks of calcium will coagulate and settle to the bottom of the bottle. Common beverage filtration will not remove the calcium from water....so even though the spirit is filtered, the calcium will pass through, and over time will react in the alcoholic liquid, coagulate, and drop out of solution. It takes a couple of days/weeks for this to occur...so the bottle may look perfectly fine as it leaves the distillery, er, I mean, mixing plant; weeks later, not so much.
The black color can be because of the dyes used to color the product....in other words, the dye colored the calcium flecks.
Just a guess. I haven't seen the bottle.