Pearl of black doubt can be useful if you are surrounded by mooks, or facing monsters with a ton of natural attacks (which all tend to be made at the same attack rating). But yeah, you really need a lot of attacks, each of which only has a modest chance of hitting you, for it to be effective.

The list of 3rd lv stances for a warblade sure doesn't seem very attractive. Think I actually stuck with punishing stance for quite a while...