I think bluntness is a very interesting idea. Why is it that sometimes social-convention-ignoring factual statements are refreshingly honest, and other times they are deeply offensive? I once had a small child touch my stomach and say "why do you have a big belly?" I thought this was hilarious and told him it was because I eat a lot. If someone older had done the same thing, though, I would have assumed some degree of malice on their part.
I've previously heard that nerds have their tact filter on backwards; whereas most people apply tact to the things they say, nerdy types tend to apply it to things they hear. This is a fun theory, but I don't think it really makes sense. Nerds hurt each others' feelings all the time, and everyone applies filters (or more generally, transforms) to the things they say and the things they hear. That's why we don't tell our parents about our sex lives, or take book recommendations at face value.
I will agree that many nerds seem to prefer bluntness over politesse, but I don't think that comes from pivotal childhood lessons about communication style. Rather, I think that it's down to the nature of their disciplines. In science and engineering, facts and rigorous thinking reign supreme. It's what I described as frustating in Mechanics and again in The apple's taste; yes, yes, you have a bunch of vague hand-wavey ideas, but how's this whole thing going to work? I need substance, because at the end of the day I'm going to press buttons on a computer and I need to know which buttons to press. If you understand that there is some essential data to be conveyed by your words, why obscure it and waste everyone's time?
Another aspect is that many nerdy disciplines involve systemic thinking. A key skill in engineering is combining many small components to make a big system, and the equivalent in science is breaking down big systems into small components to understand them. So if we think about our behaviour as small components in a big society, what behaviour is the best for the system? Is it best for everyone to waste time repeatedly encoding and decoding information in layers of euphemism and ambiguity? Well, obviously not if you think of it like that.
However, I should take the time to distinguish between bluntness and meanness here, because the latter is also tragically common among some nerdy communities. I think this is sometimes a kind of the-bullied-become-the-bullies effect, but other times it's down to a mistaken belief that being mean is more direct. For example, here's a Linus Torvalds email that includes both, and an explanation later on of his mindset. Notice the characteristic confusion about subtlety and niceness. But they're not the same thing. Subtlety obscures your meaning, niceness is just the opposite of meanness.
There's actually a useful parallel here; if what matters is facts, you don't want to waste time or effort adding extra nice words to those facts. However, you also don't want to waste time or effort adding extra mean words to those facts. Both are unnecessary for the same reason. Is the code bad? Say the code's bad. Has your friend gotten fat? Tell them they've gotten fat. But just as there's no need to hem and haw, there's also no need to swear or snear. Your intention is conveyed in these extra words you use. If you don't think your intention matters, don't add any extra words, but you can't include them and pretend you're just being honest.
Meanness doesn't come from a desire to express truth, it comes from a desire to punish or wound. A child can ask why you have a big belly and you can tell from the way they ask that they don't intend harm. For adults, it's easier to assume malicious intent, so it may be safest to signal your lack of malice with unnecessary nice words when you first meet someone (assuming you care about that kind of compatibility). However, I fully endorse the idea of dropping politeness with people you know in pursuit of a more efficient and truthful standard of communication.
But if you ever find yourself telling your friends to shut the fuck up, you may need to consider if you're being truthful or just angry.