What does the "grey envelope with a black blob on it" icon mean alongside a thread? (There's sometimes a key at the bottom of the page and it explains other icons -- like the open yellow envelope, or envelope with a padlock on it, etc. but no explanation for the grey with black blob!)

 

The "Black Blob" or black dot indicates that it is a thread in which you have made a post. Grey with a black dot means that there are no new posts since you last view it. If the envelope has 4 little lines above it, it means that the thread has more than 15 replies, or 150 views. When these threads have new posts in them, the envelope will be red/orange. Normal threads with new posts will be yellow. No matter what color, with or without the lines, if it has a black dot, you have made a post in it.