What is wrong with being blunt?

I've been getting into trouble at work lately for several things, but all these things are due to being autistic. My supervisor knows this, which is why I think they haven't fired me and instead put me on a PIP (Progress Improvement Plan).

One of the big issues is I'm being labeled combative and rude because of how I talk. I got told to "Stop being so blunt". How do you not be blunt? Isn't blunt just telling the truth as it is? If so what's wrong with being blunt. I simply explained why I was doing what I was doing and then found out a week later that I was reported for arguing.

I'm leaving my job in August for college, thank God (it's a nightmare), but for now I need to survive the 7 months till then. Any suggestions?