My observations are a bit biased as i only smoke my own blends lately. So i know whats in them(most of the time unless i forget:p) and knowing what is in them I can taste subtle differences of a few percent 2.5-5% for things like perique, latakia and orientals. And maybe around 5-10% variations for different virginias, kentucky, burley. But that's when I KNOW I made the change. taste is funny like that, once your mind knows, then it's much easier to pick things out.
As for other blind tasting of blends, i can still pick out each of the tobaccos usually, but exact ratios is much more difficult(can get a ballpark though). As for actually discerning various commercial blends, I think it really depends on how familiar you are with a blend or how much you have smoked it before. Easy to recognize an old friend or aquaintance even after years, but that stranger you met once or twice maybe not so much. Some blends have a unique aspect or topping and can be picked out easily, like the stranger that had that funny nose or bubbly demeanour.