Are you kidding. You just tell them you 'appreciate all but please I am allergic'. And tell them the reason you havn't said anything before is because you didn't want to hurt their feelings. Chances are they will love you even more for being so gracious!!!!
I agree with the above comment - signs like this are put up all the time for peanut allergies.
And if anyone gets their feelings hurt or tries to laugh it off as a joke - as stated above - remind them that YOU are the one who gets sick and suffers - and also remind them that your allergy to blueberries, while not as common as a peanut allergy - is still the same - and can cause serious, sometimes fatal allergic reactions. fortunately, your last exposure only caused a rash, but that does not mean your next exposure won't be a much more severe reaction.
With the extent of your allergy, I'd also suggest asking your doctor about getting an epi pen to carry with you in case of exposure. You may be able to avoid blueberries or blueberry products from coming into your home, but it won't be quite as easy to avoid them completely outside of your home (such as school, stores, restaurants, etc.) Much better to be prepared with an epi pen in case of emergency.
Good luck!
Hang a poster on the door with a Blueberry and the "no" red circle/slash across it.
Have a chat with this friend the next time...just say something like :
" I hate to sound rude, but I'm seriously allergic to blueberries. I'm asking ALL of my friends and family to please leave their blueberries at the door."
This would be a nice way to do...no tempers will flare...he might laugh at first, but make sure to "gently" reply that you are serious.