There is a common philosophy with regard to sleeping bags. In relation to use on the Camino, I can advise from experience that you can be walking thru 35 C deg during the day and freezing in a bunk in the evening.
I am a naturally warm sleeper but do appreciate enough bedding to ensure comfort, not perspiring.
In albergues, yes, most do provide a blanket but in March, a single blanket may not be enough. If it is of any help, here is what I do. I own an MSS (Military Sleep System) consisting of a light bag (Green), a medium bag (Black), and a Goretex Bivy (Camo). All weigh 10 lb, but I am not a soldier, LOL, and will not carry all that unless I am using a toboggan or cart. But for March, indoors, the medium bag (Black), is sufficient and can squish it in your pack last before closing, rather than rolling it up. They can be acquired on eBay anytime, one at a time or in a complete system, as needed.
Also, some have mentioned thise who sleep in their clothes. It makes sense as this provides an extra layer to help keep you warm. It also saves time in getting on the road first thing. Also, it minimizes noise when other Pilgrims are still sleeping.