Someone who constantly self-promotes their own talent, products, or business interests, to the annoyance of everyone around them. Usually an artist or musician, but sometimes a booking agent, salesperson, consultant, or other self-employed business-type.
Joe: Dude, did Dave tell you about his band's new CD?
Alan: Did he! He went on for 20 minutes about how it was going to go Platinum within a week. That blowbag!
A pack of food which should be vacuum packed but contains trapped air or a hole in the packaging.
Start opening those blowbags.