The question would be: is it/that a blue pen? And both situations could be the correct answer, a person could be asking about how the pen looks or in what color the pen writes

Im American, and I'd say that it depends on the context. So it can really mean either. In high school, my teachers had us correct our homework with red pens. This meant that the ink had to be red. But usually red ink pens will be red on the outside too. Blue pens are usually the same. When I find a pen that is not blue on the outside and it has blue ink, it may come as a surprise, bcuz most ink is black. Good question!

Here’s an American English answer — it could be both! But I would usually assume they were talking about a pen with blue ink, and since most pens with blue ink are also blue on the outside, this doesn’t come up very often. :) An exception would be if there were a bunch of pens on a table, and some of them were blue. Then if someone said “Which pen do you want?” I could say “A blue pen!” without knowing the ink color. But overall, if someone were to ask me, “Do you have a blue pen?” or say “This blue pen is really beautiful,” I would think they were talking about a pen with blue ink. One small note: I would never say “it’s blue pen,” I would say “it’s a blue pen.” Hope this helps! :)

Hi! British English here, I hope I can explain! Option 1) "What pen is it?" "It's a blue pen." In this case, both meanings would be correct. This is used when you are talking about 1 single pen. Option 2) You are writing. "What colour is that?" "It's blue pen." Here are you answering about the colour of the ink of the pen. You can also ask, "What colour ink is the pen?" or "What colour does the pen write?" if you want to know about the ink. I hope that helps!

Most of the time, it should be: It's A blue pen. (if you're talking about the pen). There is one possible circumstance in which you might leave out the article "What's that page written in? Is it green pencil? No, it's blue pen." In this case 'blue pen' describes the writing, i.e. blue ink.