Boricua" goes back centuries. The island was called Boriken by the native Taínos. "Boricua" was derived from Boriken - most likely coined by early colonial Spaniards. All those names Boricua, Borinquen, Borincano, Borinqueño - are very
old Puerto Rican terminology found in
old Puerto Rican history
books, and poems that are over 200 years
old, etc. If you have not studied in Puerto Rico and
read these words in our own Spanish literature (in school textbooks) and native poems then you might think of them as "foreign" but they are not. You are right they are not "proper" Castellano, but only Spaniards speak "proper" Castellano. These terms are actually "criollo" - developed in the island. They are
true Puerto Rican terminology.)