No marks will be reduced specifically for using a blue pen for your board exams, as long as your handwriting is legible and clearly visible.

You don't have to worry about the color of ink unless they specifically mention the color of ink in their guidelines. Only in some cases, examiners may specify the use of black ink for easier readability during scanning or grading processes.

To be certain about the rules for your particular exam, you can review the information provided before the exam.