They are jokes based on how blondes are stupid people.
e.g. A black-haired, brunette and blonde person are escaping the police. With nowhere else to go, they find three large empty sacks amidst a pile. They all hide in their own sack.
When they hear the police coming, the black-haired person goes, "meow". The police think it's just a cat.
Hearing the success of the black-haired person, the brunette person goes, "woof!". The police think it's just a dog.
Hearing the success of the other two people, the blonde goes, "PO-TA-TO".