My doctor told me I was colorblind.

This joke is a masterclass in wordplay, specifically a pun that lands with a delightful thud. The humor hinges entirely on the phrase "out of the blue." Normally, that idiom means something happened completely unexpectedly, without any prior warning. But in the context of being told you're colorblind, the phrase takes on a literal, visual meaning. If you can't see blue, then the "news" of your condition truly did come "out of the blue," because that color might literally be absent from your perception, or at least indistinguishable. It's a clever twist that leverages both the figurative and literal interpretations of a common saying.

Color blindness, or more accurately, color vision deficiency, is a real condition where people have difficulty distinguishing between certain colors, most commonly red and green. It's often genetic and affects more men than women. The idiom "out of the blue," on the other hand, has a less colorful but equally interesting history, believed to have originated from nautical terms, referring to something appearing suddenly from the clear sky or ocean. This joke brilliantly smashes these two disparate concepts together for a quick, unexpected laugh that's anything but monochrome.