Hello !

Mostly black ball pen is used to fill OMR sheet. But if you can use blue ball pen also, no issue. First read the instructions given in question paper carefully before filling omr sheet. Black ball point pen gives more contrast and more area is filled faster. And OMR is all related to filling circles. That's why black ball point pen is preferred. But if you have used blue ball point pen, its certainly no issue and you don't have to worry about anything.

Best of Luck !