There's a difference between honest and blunt. Very few people react well to direct criticism and go on the defensive.
There are a few tricks you can employ to soften the blow:
Framing a problem in a way that allows the other person to feel like the can step up and do something positive gets better reception than framing it like they should step away from doing something negative. People want to see themsevlesthemselves as the heroes not the villains.
On a macro level balancing your criticism with positive observations (and leading with them) can help put people at ease and make you appear less of a pessimist/complainer/picky and more thorough.
For specific personal criticism the sandwich strategy can also helpful. It's very similar to the one before but aims to improve who your opposite feels. I didn't read the previous link. It does raise some concerns in using the strategy but also refers to an approach that is slightly different from what I have learned. The article uses the formula "some good feedback" + "some negative feedback" + "some good feedback"
"some good feedback" + "some negative feedback" + "some good feedback". And if used clumsily I can definitely see how this would backfire ("Hey X, I appreciate your punctual arrival every day, you're an absolute asshole to work with but I do commend you for the long hours you put in!"). I've learned that for a good sandwich the formula should go more like this: "praising a positive aspect of the problematic behavior" + "bringing up the negative" + "offering a positive option for improvement"
"praising a positive aspect of the problematic behavior" + "bringing up the negative" + "offering a positive option for improvement" ("Hey X, I really appreciate how open you are to help your fellow workers with their computer problems, unfortunately your cologne is quite a bit too strong for many people, if you could tone it down some your help would recieve even more appreciation.") But, yeah. The article is correct: if you use this as a cheap trick it's probably going to backfire. But so is smacking people over the head with just the complaint. It is worth considering whether you can pull this off in a genuine and sensible way.
I agree with others that it's difficult to judge the situation, whether your boss will be open to what you have to say. But I'd say it's probably better to assume good intentions and do what you've been asked to do (give honest outsider feedback). If that isn't actually wanted you're in trouble anyway so might as well find out early...