Honesty is a topic that is frequently explored in ethics, because the question of what exactly honesty is is central to ethical discourse. If one takes the honesty to be absence of the intention to misrepresent or deceive, then a discussion about the nature of honesty hinges on the philosophy of mind and is related to the legal notion of mens rea. Furthermore, one can stake out both an action plan and an attitude towards an act. From WP:
Intentions are mental states in which the agent commits themselves to a course of action. Having the plan to visit the zoo tomorrow is an example of an intention. The action plan is the content of the intention while the commitment is the attitude towards this content. Other mental states can have action plans as their content, as when one admires a plan, but differ from intentions since they do not involve a practical commitment to realizing this plan. Successful intentions bring about the intended course of action while unsuccessful intentions fail to do so. Intentions, like many other mental states, have intentionality: they represent possible states of affairs.
The point, therefore, of calling a lie broadly as an action with an intent to deceive is to move the burden of the examination from absolutist moral notions about actions towards a situational ethics. The difference is embodied in such a simple notion as "telling untruths is wrong" should be "telling untruths is generally wrong". If the untruth saves a thousand lives in the act of war, I suspect, most people would consider that an ethical act.
Now, the phrase "if I can be honest" isn't about being honest at all. It's generally a pragmatic linguistic construction to signal to the listener that something that will make the listener uncomfortable may be said. So, now, instead of being in the epistemological domain of characterizing statements in regards to truth-conditional semantics ("It's true! It's false, and you knew it, and now I question your character!") we are more squarely in the domain of ethics objectively. Thus, it is appurtenant to ethics insofar as through language, you are "hurting the emotions" of another which is independent of the speakers intention. Generally, unlike the question of honesty which pivots on the truth-conditional appraisal of utterances and the intent of the agent constructing them, implicature is more often the focus. That is, what are the greater meanings and implications involved above and beyond the statement.
Bluntness, therefore, might be seen as turning a blind eye to using implicature to be vague and explicitly constructing utterances that would allow an agent to otherwise avoid meaning, carry negative connotations, or miss inferences in grammatical constructions. Hence, bluntness often has to do with explicitly conveying normative notions. Let's say a husband gets caught eating cookies he's been asked not to eat.
Dishonest: "I didn't eat the cookies that you asked me not to eat." crumbs on his beard
Honest: "I ate the cookies that you asked me not to eat." contrite expression
Blunt: "Damn right I ate the cookies you asked me not to eat." rebellious face
The difference between dishonest and honest is about the truth condition of the explicit language. The tone of the honesty is about the listener's emotional response.
Another example of a husband talking to an overweight partner:
Dishonest: "You don't look a drop over 100 lbs in that dress!" to 200 lb partner
Honest: "You look like you are about 200 lbs" matter of factly
Blunt: "You're fat." runs out of the room in fear
Again, the truth-condition is the difference between the truth and the falsehood, and again, the negative connotations associated with the term fat are sure to trigger a negative emotional response in the listener.
So, from a philosophical perspective, what is at stake in both actions (being honest and being blunt), is an ethical question about the effects of the utterances. In the immediate, is it right to say things that would intentionally upset or sadden someone else? Will the act create bigger ethical dilemmas? Will the words cause a divorce and split a home with children? Is shaming a fat person into becoming healthier moral if it causes them to lose weight and avoid a heart attack that kills them? And of course, deciding what is ethical or not for a thinker can be a convoluted affair.