I have also seen that myoglobin is not the same as haemoglobin, so what is the source of myoglobin? Does it come from the blood?
Myoglobin is a protein made in muscle cells. It is similar to hemoglobin, a protein made by blood cells. The answer you linked helps explain the difference, which it sounds like you understand. The source is simply the ribosomes in the respective cells that express that protein. The color in both blood and uncooked or rare meat is due to the heme group in hemoglobin and myoglobin respectively.