The chemical element that is commonly blue is copper. When copper is exposed to oxygen and moisture, it forms a blue-green patina on the surface, which gives it a blue tint. Some copper compounds, such as copper sulfate or copper carbonate, also have a blue hue. However, it's important to note that not all blue substances are made of copper, as there are various other chemicals that have a blue color, such as cobalt or indigo.
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