Do blood group and blood type refer to different classifications of blood (A, B, AB, O vs. something else), or are they used in different contexts to refer to the same thing?
Blood group and blood type are largely synonymous, but are often not used synonymously.
this article discusses blood groups, and introduces the act of blood typing as well.
Blood group tends to be used when discussing the A, B, AB, O, and Rh blood cell surface antigens.
Blood type tends to be used when discussing individual members of a blood group, e.g.
In addition to the ABO and Rh types are many more types which aren't contained in groups, such as H (or Bobmay antigen) and Le -/+ (Lewis) types. In the article about the child with a rare blood type, his type is found in only 14 people in Pakistan.
They mean the exact same thing. I would guess that they are used in different contexts but could be used in the same context.