This is my main fear with radical honesty, since it seems to promote thinking that negative thoughts are true just because they are negative. The reasoning going 'I would not say this if I were being polite, but I am thinking it, therefore it is true' without realizing that your brain can make your thoughts be more negative from the truth just as easily as it can make them more positive than the truth.

My own (very limited) observation of trying to be radically honest has been that until I first say (or at least admit to myself) the reaction of annoyance, I can't become aware of what lies beyond it. If I'm angry at my wife because of something else that happened to me, I usually won't know that it's because of something else until I first express (even just to myself) that I am angry at my wife.

Until I actually tried being honest about such things, I didn't know this, and practicing such expression seemed beneficial in increasing my general awareness of thoughts and emotions in the present or near-present moment. I don't even remotely attempt to practice radical honesty even in my relationship with my wife, but we've both definitely benefited from learning to express what we feel... even if what we're feeling often changes in the very moment we express it. That change is kind of the point of the exercise: if you've completely expressed what you're resenting, it suddenly becomes much easier to notice what you appreciate.

I think that even Blanton's philosophy kind of misses or overstates the point: the point isn't to be honest about every damn thing, it's to avoid the sort of emotional constipation that keeps you stuck being resentful about things because you never want to face or admit that resentment, and so can never get past it.