I see Scotch is often labelled as single malt or blended, but never both. What, exactly, is the distinction?
We need to work up a few definitions before we get to the final answer here. So without further ado:
Single Malt means that the whiskey has been made using ONLY malted barley and nothing else and has been distilled at a single distillery. This is to distinguish it from:
Grain Whiskey, called Single Pot or Pure Pot when it's Irish, is still whiskey from a single distillery but can have other grains in it, such as rye or unmalted barley.
Single Malt + Single Malt = Blended/Vatted Malt
By mixing single malts from different distilleries you get a Blended Malt or Vatted Malt whiskey.
Single Malt + Grain Whiskey = Blended Whiskey
When you mix Single Malt with anything besides more Single Malt you get regular old Blended whiskey.
A lot of these terms only really apply to whiskies produced outside of the US, namely Scotland and Ireland, but Japan's whiskey-producing traditions are based primarily on Scotland's.
In the US the whole malt thing goes out the window since the legal requirements only specify "Majority" grains, IE American Malt Whiskey only needs to be 51% malted barley. Rye Malt is 51% malted rye, etc. But the US terms also put a lot of other restrictions on the length of time the whiskey was aged and the alcohol proof at after distillation, when put into barrels, and when bottled. But that's a different question.