Sounds like parts of the toilet ring seal that prevents leaks around the toilet base. BUT: typically they are black or dark gray. It may be construction residue from manufacturing ?????
If everything works as normal, I'd consider it "black tank waste", keep using the toilet, watch for more of the same and not worry too much about it.
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John
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS
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