Black pens have that 'oooh' factor.... it just looks more sophisticated and smarter..... well it does to me and the fact that if you want to have your paper graded as a band 6, you should do what the example responses do and write in black pen.
And for exams, always carry in a Bic pen with you, just a normal clear plastic one which you can get from any woolies/coles/newsagency because they will always work and is probably the best pen to have as a backup. Any company which can make reliable pens and lighters has my approval
I always go for those Uniball "Eye" black needlepoint pens. They cost a pile but it doesn't matter, anything's worth the difference between bands 5 and 6 right?