What youre doing isnt honesty or bluntness. People appreciate honesty and bluntness when they ACTUALLY appear, but thats not what youre doing.
No see the blunt response, the response I give when i dont want to talk is this:
"Hello! I'm sorry, ive had a really bad day today, i really dont feel like talking to anyone right now, i hope thats alright. I hope youre having a great day though! I just need some time to sit and think."
People appreciate the honesty, because that is actually honest. What you, and LOADS of other people who claim to be "blunt" do, is express what you think in the most cowardly way possible as such to avoid giving any sort of emotional ***** in your armor. Like delivering the message without giving anything else away. Youre still holding back, so its not really blunt at all. Youre afraid to speak to people for whatever reason, and dont wish to admit it to them. So in that sense youre lying anyway, the honest answer is the sort i gave. Thats not necessarily a bad thing, but call it what it is.
And you know what, you dont HAVE to be blunt like im blunt, you CAN lie, but your issue isnt with lying since youre sort of doing that anyway by omission, your issue is paradoxically you want to not care about anyone but still care what people think. You cant have both. Id advise giving strangers a chance and not trying so hard to be aloof and uncaring. Its really not as fun.
EDIT:
Because you wanted help and I like you (as i extend a basic degree of friendlyness for all strangers such as yourself!) ill give you some ideas of what to say.
"Hello, Yeah thats very nice/its been great/one sentence to be polite." followed by
"I'm not massively in the mood for a chat right now, sorry, hope youre having a good day though"
Then walk away. This is only if you WANT to be seen as polite. And thats your choice. Otherwise id say what youre going with is extremely rude.
That said, my actual JOB is to make small talk with (unfortunately very sick) strangers at the local hospice. A lot of people like small talk when they are upset and stressed, its them reaching out into an uncaring world for some support. Like you are doing right now. You gotta face facts and accept that in your view reaching out to a stranger for help/support should be met with denial and ignoring. Yet thats exactly what you did OP. Youre not so different to those people you dislike really. Its worth thinking about.