It depends. It can look OK as a casual variant or it can look terrible. Since we don't know how the blazer exactly looks we can only guess.
Dark gray blazers can and do look nice with dark jeans. Nice autumn/winter casual outfit. The only problem here are the pinstripes. Since StyleLion mentioned that the pinstripes are closely matched it can look awkward.
The best is to try. Trying to make pants that look alike blazer will not work.
Besides I do not wear pinstripes. I am tall enough :)