A Blue Dog Democrat is so-called because of the character Dino on the cartoon television show The Flintstones. Dino was a blue dog, and the letters DINO stand for "Democrat In Name Only."
Blue Dog Democrats are also sometimes called "Reagan Democrats." They are conservative, rather than progressive, and have a tendency to align themselves with GOP policies in matters of taxation and spending.
The Blue Dog Democrats are standing in the way of true healthcare reform.