Is it okay to wear a dark grey blazer over black trouser along with a white shirt for an interview?

If you're asking about fashion choices, a black blazer over grey trousers would probably be better. As for what's appropriate for an interview, it's usually most important that you appear neat, clean, and respectful. However, some careers might demand more attention to style and fashion. Besides the obvious ones (fashion industry jobs, clothing stores, etc.) high-paid lawyers, consultants, and executives need to be very well dressed. Appearance can be important in some jobs. If you're applying for that sort of position go to a high-end department store and ask a salesman to outfit you. Be prepared to spend hundreds of dollars.