hello lakshmana!!

you can use either colour of pen but please prefer black ink pen while bubbling the omr sheet.

black ink pen has dark impression and while filling omr it is better to use black ink pen if it not mentioned with which colour pen to bubble the omr.

all the very best 

Hey Lakshmana,

Always prefer blue pen in any examination if you are not provided any prior directions by the invigilators present during that examination or in the case you have not been provided any pen for that particular examination. I hope you have got the point.

Good luck !