i'm a Nigerian student studying in Istanbul, Turkey. I've been granted a one year national visa to Italy but i'm to enroll with the ERASMUS study program for a semester in Barcelona. Do i need to obtain a study schengen visa to proceed with my program or will the national visa suffice?
To stay for more than 90 days in Spain (+ all other Schengen countries excluding Italy) you need a study visa from Spain.
It is not clear what you have been granted the Italian visa for, but it is advised against using it for a completely different purpose than what you stated in your application. Studying in Barcelona instead of Italy would be such a case and it should be voided.