All color is bile. Normal bile is a nice nut brown. Other variations include yellow, green and browns to deep black, which are old bile, which is toxic waste and needs to come OUT!
This trip my outpouring of black bile happened absolutely spontaneously, which is a bit rare. Looks to me like something tripped my liver, because there were other "livery" indications, like what we liver flushers call "chaff", probably that "particulate matter" you refer to. Fasting is a wonderful rest for the liver, and you can bet it is busy busy busy offloading trash, which is why it is so important to keep our little colons April Fresh every fasting day!
Today I feel wonderful, weak and tired but happy. Obviously I needed that!