Why does the word written in blue ink turn black?

The main component of black and blue ink is tannin. It is a non-green substance that is neither black nor light green. Of course, such ink will be unclear, so organic dye is added to it. So the ink will appear more blue.

So when you first use blue and black ink to write, at first it is blue, which is the original color of the ink. But over time, the phosphate in the ink will react chemically with oxygen in the air and turn into iron. This is a black precipitate, which changes the preceding letter to black.

There are some people who have finished writing ink often forget to cover the cap of the ink bottle, doing so has 2 should not. One is that the amount of water quickly evaporates, the more and more the ink is depleted. The second is the phosphate substance in the ink that will react with the oxygen to become iron tannin. This substance precipitates. As a result of the ink being precipitated when dots on the pen, the residue of the ink will clog the pen, making it impossible to write words.