The word "ink" comes from the word "black"because the first ink made with the use of coal or soot, and they really had a black color. However, let's look at the dictionary S. Ozhegov " and N. Shvedova:
INK-Il. Ink liquid to the Scriptures.
As you can see, the word "ink" has changed its meaning and refers to any paint that is used for writing. Meanwhile, in our time with this purpose, use the coloring liquid is not only black, but also a variety of colors - blue, red, green, purple and so on, But by tradition they still called INK.