my all time fave is:
What do you get if you turn a blonde upside down?
A brunette with bad breath!
And to show "blondes are stupid" jokes have some basis in reality, a blonde friend went out for the day to another city, but left her keys. She phoned and we explained she should phone again before she set off back so we could wait in. Later, phone call comes so we wait in for her, she arrives 10 seconds later. She'd only phoned from the payphone 20 yards from the house...
She was also asked by a guy in a club if she was on e-mail.
She said they made her feel really good.
So, whilst not all blondes are this level of dim, they do exist.