The simple difference is the use of grain whisky as the base. Daniel Coffey (debatably) invented the colum still which allows a faster, higher volume but argubly lower quality distillation. This allows mass production of an inferior product - very few distilleries have won any real plaudits for their single grain releases. The grain whisky then requires the addition of various single malts in order to provide complexity, depth, structure etc.
I think it would be unfair to suggest blends are not as good as they can be terrific bottles but the cieling is probably lower. Some of the blends can make incredible releases and sone of the more 'standard' levels are also impresive - Johnnie Walker Blue, Grant's 25, Ballentines 17 etc. These sort of blends will not hold their own in a direct comparison with similar priced malts as they are a different offereing but will definately stand up on their iwn and be counted.