Bluntness, or being overly transparent is a character "vice of excess" stemming from what would otherwise be a character virtue in the form of honesty. It is intellectually narcissistic, which is to say that it contains either the intellectual narcissism of ignorance or prejudice. One may be ignorant of how much of your opinion of themselves that another person wants to hear from you ("TMI" - too much information), overestimating their openness to feedback (poor boundaries), or making them uncomfortable. Bluntness may be prejudicial against those who value their privacy and want to be left alone. In this case, bluntness can verge on the "obnoxious."
Someone who is "blunt," is by definition, forcing their private opinion of you to the point of losing the courtesy, diplomacy, and good boundaries of retaining mutual respect (and the giving of voluntary access to others about your nature.) However, all human beings communicate on a spectrum between being overly transparent bluntness, and too chicanerous to gain the trust of their audience. This highlights the central psychological skill that counters vices: maturity, which contains the Observing Ego to scan the environment for its composition and needs, matching that to what we endeavor in that social environment.
As a vice which makes partnership within relationships fail to get to goals, and intellectually unattractive, consider this vice to be negative in many ways, including in sexual attraction, since it is "brutal honesty." Radical honesty can go too far, since every lasting romantic partnership needs its ample serving of "little white lies" that are a part of courteous diplomacy.