I realize that the Kingdoms weren't officially integrated until the 18th century; that they were instead, held in a personal union up to that point. But I'm curious to now - was the term "Spain" used to describe them prior to that point? Were Isabella and Ferdinand actually ever referred to as the King and Queen of Spain by their contemporaries?
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It is said in the case of Spain that the identity of "Hispania" was forged abroad. There was the historic "Hispania" encompassing the entire peninsula, which today comprises Spain and Portugal. The roots of the modern definition started to grow during the rule of Isabella and Ferdinand. As they came out victors in the War of Castilian Succession, they forced a settlement on Portugal that Portugal could no longer claim to be inheritor of the Hispania identity of old.
As far back as 1500, cries of "Santiago, España!" were used by Spanish soldiers in the tercios in Italy to signify their place of origin on the peninsula. But the identity was not uniquely based on ethnicity nor place of origin, as Italian soldiers in Spanish employ used the battle-cry "Santiago, Spagna!" as they fought in Rhineland! But it was clear to what they paid tribute to, and the use of the term stuck.
The use of "Spanish" as a shorthand was forged by the explorers, adventurers, and soldiers abroad. Conquistadors started to identify themselves as "Spanish" in contrast to the natives.
So in the case of Spain, its national identity was formed as a shorthand on one hand, and more importantly as a response to "other"-ness as they ventured outside of the peninsula and outside the continent.
-- M. J. Rodriguez-Salgado, Christians, Civilised and Spanish: Multiple Identities in Sixteenth-Century Spain, Trans. Royal Historical Soc., Vol. 8 (1998), pp. 233-251.