Not quite home-blended, but Penfolds seems to try some far-blend assemblage of its own!
One trick that I remember being employed, is to take for example 2 bottles of an unremarkable Sauvignon Blanc and mixing them with a much better one. The result being that you now have 3 bottles of good wine.
Someone told me that they had used innocuous Soave with a bottle (sorry can’t remember) of better, apparently they were then in Harry Potter Territory!!