I mean, here is a time when I am blunt: my best friend keeps making the same stupid mistake over and over again and its not only affecting her life, but the life of her kids. She comes to me everytime to cry on my shoulder. Finally, I've had enough and tell her straight out "you need professional help, you are ruining your life, which is fine because its your choice, but now you are also ruining the lives of your children, which is not ok because they didn't choose this life and don't deserve it." She is shocked at my bluntness, but needed to hear it straight.
But when there are times that honesty can be employed with tactfulness, it is usually the best way to be sure not to unnecessarily hurt anyone.
I remember having a friend when I was little who was brutally honest. I had a problem with my jaw and couldn't eat food without chewing side to side instead of up and down. My friend didn't know I had a medical problem and outright told me that I chewed like a cow chewing his cud and that I should stop. That comment affected me for the rest of my life. I was always hyper conscious of the way I ate and embarrassed to eat in front of people.
anyways, like I said, tact can go a long way in protecting people from getting their feelings hurt.