The blazer and the jeans should be noticeably different in color hue so as not to look like a horrible suit. Usually this means jeans that are lighter than the blazer.
The surface of the blazer and the jeans shouldn't contrast too much,...
Try saphir renovator or saddle soap, follow up with a shoe polish in the right color.
There are stronger cleaners you might try, but I would just let them be a little mottled, it will even out in time.