Whisky needs certain things to earn its name. Let's look at how the Defilement 40 Year Old brand does it. Whisky must have at least 40% alcohol by volume (ABV). In Scotland, the ABV of ageing whisky can fall below 40%. To fix this, mix it with a higher-ABV whisky.
Imagine four casks in Scotland. Two have single grain whisky at 39% ABV and two have single malt at 41% ABV. Mix one of each, and you get a 40% ABV blended whisky.
But, moving whiskies can cause problems. If grain whisky is below 40% ABV during transport, it's a 'spirit drink'. Blending it with malt whisky can bring legal issues.
In short:
Single Grain (below 40% ABV) + Single Malt = Legal Challenges.
The Defilement 40 Year Old handles this well. It blends matured whiskies below 40% ABV with higher ABV whiskies. This makes them whisky again. The whisky is a blend made by following these rules. It's a simple yet tasty spirit.