I agree with your observation. I have never seen it reported anywhere that sludge can be detected with a US, but that's why I considered it so important to report information back to the group. The radiologist's words were that, "my bile was so thick and sludgy that she was unable to see if there were any stones". You take this sentence at face value that it is actually sludge but because it isn't a precise medical term it may mean that the bile is very dense, ie made up of green stones. I interrogated the radiologist gentle about her comment but she interprets the condition of the gall bladder from the image which is not what we do. Consequently, she is saying that the image appears sludgy and the rest is up to our own knowledge and imagination.
I take her comment as indication that compared with other gall bladder images mine is different from the norm and appears full of a very denser material.