Here's one that's not as crude and filthy, but oh well. Originally a blonde joke, but the color of your hair proves nothing, so now it's just a joke about a slow-witted woman.
This lady's house catches fire and she calls the fire department. "Help! Come over quick! My house is on fire!" she says.
"Okay, what's your address?" The guy on the other end asks.
"...I don't know." she says.
"Well how do you expect us to get to the fire without knowing the address?!"
The lady sighs, "Duh! THE BIG RED TRUCK!"