I told my doctor I had a bad case of the blues.

This joke is a classic example of wordplay, specifically a pun, which relies on the multiple meanings of a word or phrase to create humor. The phrase "a bad case of the blues" is commonly understood to mean feeling sad or depressed. The humor arises from the doctor's deliberate misinterpretation of this idiom, taking "the blues" not as a state of mind, but as a literal overexposure to the color blue.

"Having the blues" has been an idiom for low spirits since at least the 18th century, possibly stemming from the idea that anxiety could produce a livid skin color. When a patient approaches a doctor with such a complaint, the expectation is a diagnosis related to their emotional well-being. The punchline hilariously subverts this expectation by having the doctor offer advice more suited to an interior decorator than a medical professional, suggesting the patient simply stop painting their house. This unexpected, literal take on a figurative expression is what makes the joke land.

The comedic twist lies in the doctor's absurd advice, turning a seemingly serious medical consultation into a lighthearted misunderstanding. Doctor jokes, in general, have a long history, often playing on the doctor-patient relationship and using wordplay to lighten the mood. This particular joke thrives on the cleverness of its pun, forcing our brains to quickly shift between the two meanings of "the blues" to appreciate the silly scenario it paints.