The small black particles are normal on stationary trainers. This also happens when on the road or trails, one just does not see it.  It is the hard-wax components of Squirt shedding from the chain.  Excess wax and dirt trapped sheds via this method.  You may find that more wax accumulates on the chain of your trainer – this is due the chain not vibrating/shaking enough to promote shedding. Our biodegradability rating is therefore important as it would end up in the natural environment.