Today's episode featured Steve's neighbor Miranda and her dog Magenta, who needed to get glasses. Magenta looks like Blue, only she's pink. Magenta doesn't speak either! This leads me to believe that, instead of that Blue does speak with a speech impediment, either that, in the Blue's Clues universe, dogs can't speak, or animals that are owned can't speak.
Ergo, it's all a very clever commentary on property and/or slavery and/or animal rights. And there you are.