Normally the things people call "stones" here, float due to their properties. A stool can also float but depends on diet, people that have high fibre diets find usually their stools float. You can discover this for yourself by using Pysillium husk for say three days, about a tablespoon three times a day with a good diet.
I'll warn you now be prepared to have fun trying to flush them!! :-)
The items you mention possibly are "stones" but they are stuck in your stool and since it is a sinker the stones sunk with it. You could've tried breaking it up and then you probably would have found them to float. Having black in a stone is indicating usually that it has got lots of bilrubin in it which comes from your bile. Stones commonly have a mixture of compounds, the nasty ones have a calcium hard shell and sometimes even deeper than a shell these are typically what ultasounds always pick up. Any soft ones without calcium have poor reflective qualities so US usually doesn't pick them up.
Either way flushing helps clean things out, to help this cleaning process the Epsom Salts would have helped you as it loosens everything up due to the increase in fluids being flushed.