Using a grayed-out padlock has me questioning each time I look if its actually a private message or not. Absense of padlock = public comment, presense of gray padlock = private comment. I would rather gray padlock = public comment, blue padlock = private comment.
I agree that it is super difficult to notice the private comment padlock icon.
I really, really, really hate the fact that the padlock is grayed out and not obviouse. I cannot tell you how many times I have started to move or respond to a comment I thought was made publically only to hover over the padlock to see it was actually made private. Not sure why it is so hard to change the color to make it more obvious.