I walked from Leon to Santiago last august and I only brought along a silk liner which I came to regret ad few times since I was very cold during three nights. Temperatures vary a lot due to variation in altitude but also in isolation. I especially found old cosy stone albergues to be very cold at night time despite a lot of people sleeping in the same room.
Most places did have blankets but not ones that I felt comfortable using. This time I am going to bring a summer sleeping bag when I rejoin the Camino next month. I will probably be very warm some nights but I definitely prefer that over freezing my but off!
Buen Camino.