Will a bone marrow transplant change one's blood type?
Or is the donor blood type matched with the person before transplant?
Bone Marrow transplants are extensively matched in order to prevent rejection. Current matching methodology is HLA matching: Stanford Children's Hospital Guidelines
However, because the HLA matching doesn't utilize blood-type and the recipient's marrow is destroyed in the process, then if the donor's marrow produces different red blood cells - then so will the recipient with time:
What's interesting is that blood-typing is not be expressly necessary with the modern bone marrow transplant process. The danger of mixing two antagonistic blood types in vivo is that the recipient's immune system attacks the foreign red blood cells resulting in body-wide rejection. However, because the immune system's response capabilities are decimated by the destruction of the recipient's bone marrow (usually accomplished via chemotherapy or radiation), it can't mount much of an attack in the first place. That's not to say the body can't mount an attack, or that the donor's tissue can't mount an attack on the recipient (Graft-Versus-Host-Disease [GVHD]) - but the better the HLA match the less likely issues are to arise, which is why we do the more accurate and relevant HLA matching instead of blood-typing.