A person who posts on a blog thread, in the guise of "concern," to disrupt dialogue or undermine morale by pointing out that posters and/or the site may be getting themselves in trouble, usually with an authority or power. They point out problems that don't really exist. The intent is to derail, stifle, control, the dialogue. It is viewed as insincere and condescending.
A concern troll on a progressive blog might write, "I don't think it's wise to say things like that because you might get in trouble with the government." Or, "This controversy is making your side look disorganized."