I know what you mean. I've run into it at times, because the person in question was such a large part of my life it's hard to never, ever, mention her. There have been times when I get the blank look, as if the mere mention of her name makes someone think I'm "hanging on" to something that happened ages ago. I've been over it for ages, but it's not possible to talk about a certain period of my life without mentioning her name, because she was pretty much always involved in whatever it was I'm talking about.
In your situation in particular, you really need the people around you to understand what a big, big, unimaginable thing you're living with. Someone needs to tell whoever does that how they can best be supportive of you. Of course, in fairness to whoever they are, maybe the loss was unimaginable for them, or else maybe putting themselves in your place makes them feel like they can't offer anything to say. Maybe they hate knowing you have it to deal with, or maybe they feel if they tried to say something not being in your place would mean they would say the wrong thing.
I don't know how long it's been for you, but sincerest condolences. (Is there any way you can talk to whoever this person is and ask why s/he is uncomfortable when you mention him? Maybe it would help to get everyone's feelings about the whole "talking situation" out in the open.)