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Surprise surprise, it is actually blue (lapis lazuli, the minecraft resource). The blue that appears to be everywhere from the sky to the ocean. However none of that blue is actually really blue (as in there isn't a pigment, an organic compound that gives a characteristic color to plant or animal tissues, that causes the blue). Easiest way to prove it is to just collect some ocean water and look at it.

There isn't any actually "blue" animal in nature except for the obrina olivewing butterfly, which is the only known animal in nature that produces blue pigment. The rest of the animals, like for example the blue Jay, look blue because for example their feathers are built to cancel out every other color until only blue remains. Everything you thought to have have known about blue flowers is also false. There is no true blue pigment in plants. The blue color is caused by a red pigment known as anthocyanin.