The term "tea bag" was photographed on a sign at a protest ("Tea Bag the Democrats before they Tea Bag you"). That sign seems to have been aware of the double-meaning. Fox News started using the term "teabagging" shortly after that, seemingly without really understanding the meaning. Salon.com was probably the first to publicly point out the double meaning explicitly, MSNBC followed shortly with the mockery.
re: blue dog, according to wikipedia, " "Blue Dog Democrat" is derived from the term "
Yellow Dog Democrat." Former
Texas Democrat Rep.
Pete Geren is credited for coining the term, explaining that the members had been "choked blue" by "extreme" Democrats from the left"