What is blending of gasoline?

Blending is a process of mixing gasoline or petrol

with cracked and reformed gasoline, alcohols, benzene etc to increase its octane number and anti-knocking characterize.

Diesel oils are not blended.
The reason-knocking tendency of cracked gasoline is less

Cracked gasoline is rich in branched unsaturated hydrocarbons. The molecules of cracked gasoline are more compact than that straight run gasoline. Compactness of the molecule increases its anti-knocking properties. Hence, knocking tendency of cracked gasoline is less.