Can O’ Worms.
Not a blend (though it would be a decent name for one), but this thread.
My understanding is as follows (it’ll be wrong or arguable, but it won’t be far off).
Most whisky is blended.
Single Malt is a blend of whiskies from a single distillery. If it has an age, they will be of the same age. Normally there is water added during the blending – if it’s cask strength there wasn’t.
Single cask is unblended but may have added water.
Single grain whiskies also exist, though I forget any examples.
Blends may be malt or grain and generally refer to blending more than one distillery. The good blends are as good as the good singles, and the best are excellent. They also cost as much.
All distilleries are the oldest, newest, smallest, highest, lowest, most remote etc.
Of course like all Scots mythology, most whisky lore is subject to change without notice.