The Toggle Comment command in the Block menu combines the functionality of Block|Comment and Block|Uncomment. On each line in the selection, it will attempt to remove the comment characters exactly like Block|Uncomment would. If there are no comment characters to be removed, it will add them on that line just like Block|Comment does.
If you did not select any text before using the Block|Toggle Comment command, or the selection does not span more than one line, Block|Toggle Comment will add or remove the comment character at the start of the line containing the text cursor. The text cursor is then moved to the next line. If you've assigned a keyboard shortcut to Block|Uncomment, you can quickly uncomment a bunch of lines this way.
The advantage of Block|Toggle Comment is that it allows you to use the same keyboard shortcut for both commenting and uncommenting lines.