A blendstock is a component blended into a finished fuel rather than sold on its own. The finished product you pump is built by mixing a base fuel with one or more of these components.
Most fuels are not a single substance. They are a recipe. A blendstock is one of the ingredients in that recipe, a component added to the base to reach the finished fuel. On its own a blendstock is usually not a ready-to-use fuel.
The clearest examples are renewable. Ethanol is the blendstock blended into gasoline to make the E10 sold at most pumps. Biodiesel is the blendstock blended into diesel to make B5 or B20. The base fuel plus the blendstock equals what the customer buys.
Blendstocks matter in the trade because they are often handled, taxed, and tracked separately from the finished fuel. Many terminals store the base and the blendstock apart and mix them as the truck loads, and the renewable blendstocks carry credits that ride on the volume blended in.
So when someone in fuel says blendstock, they mean an input to a finished fuel, not the fuel itself. Knowing which is which keeps the gallons, the taxes, and the credits straight.
In useThe terminal stores gasoline and ethanol separately and meters the ethanol blendstock into each truckload, so the fuel is finished as E10 right as it loads.
See also Ethanol blend, Denatured fuel ethanol, Biodiesel