Interesting, and I have no answer for it. The maple charcoal filtering that occurs is through a vat which precedes any filtering that occurs during bottling.
However, it is not uncommon for barrel char to occur in bottlings of American whiskey -- bourbons George T. Stagg, Booker's, William Larue Weller, et al -- that are otherwise unfiltered.
I really can't see how that can happen with Jack Daniel's. However, export barrels are processed differently than those intended for domestic/U.S. consumption. I think it is safe to assume that any 'black particles' that occur are either from the original barrel or from the maple vat through which JD is processed.

no need to worry. is simply some charcoal that has managed to make its way through the filtering process from the barrel. i myself work in a distillery that actually uses ex jack daniels casks, and you would not believe how much charcoal comes out when it comes round to emptying!!