A cognac is called a blend when it is made of grapes of different Cru’s or of different types of grapes or of different years (different vintages).
When a cognac is made of grapes from one vineyard but of different vintages, it is not often called a blend but rather a ‘single cru cognac’.
If a cognac is not a blend it is usually named after its cru. For instance: a grande champagne, or a borderies etc. And when it is also made of the harvest of one year it is aso called a vintage.
Bouilleurs de cru (winegrowers that only sell their own products) usually make a single cru and very often it is a vintage as well. The big firms nearly always make a blend, eighter of different cru´s or of different years. Very often they buy their cognacs from bouilleurs de cru of several different cru’s.
Since some years now there is a tendancy towards making more vintages as a way to oppose the competition of whisky.