Non-multiple choice questions, your teacher asked you to use a blue pen? I may be afraid that if you use black, it will make it too black and dirty and affect the paper.

Nowadays, black pens are generally used, as well as pens for exams. Don't buy anything too thick, just 0.5. The key is that the font should be neat, and it doesn't matter if the font doesn't look good. It's important for the teacher to see that you are writing seriously. This is a question of attitude.

You'd better listen to the teacher. Since they told you so, it must be reasonable.