So I recently had the occasion to give a loved one a priesthood blessing related to a cancer diagnosis, and as I've contemplated mortality and healing I've started having a lot of questions which I've never thought about before. After that blessing I went and read Elder Oaks' talk from the most recent conference called "Healing the Sick". The takeaway point, I think, is the following:
From all of this we learn that even the servants of the Lord, exercising His divine power in a circumstance where there is sufficient faith to be healed, cannot give a priesthood blessing that will cause a person to be healed if that healing is not the will of the Lord.