I know it sounds distasteful, but getting any 'still in the colon' released 'stones' out can make it worthwhile: Try an enema if you have the same type problem next time.
Before liver cleansing, try to eat light the day before, then do a saltwater flush the morning of your cleanse to help clear the way for the 'stones' later. The 'stones' sitting in your colon can make you feel a little ill or bloated, so giving them a quick shot out is a major help.
'Stones' can come in many different colors, from a dark almost black green to emerald and lime green as well as brown, tan, and rust. Read in another post that the darker the color the older the 'stone', and the more brownish ones are more likely from the gallbladder. Were the 'black things' kinda rounded shaped and floating - if so, they could have been old 'stones'.
You can usually find a bag of long bamboo skewers at the grocery that you can use to poke around in the bowl at a distance to get a better idea of what's being released.