Would a refinery that receives RBOB from another refinery and blends oxygenate(s) with that RBOB to make RFG also have to register as an oxygenate blender?

Yes. Under § 80.2(mm), an oxygenate blender means "any person who owns, leases, operates, controls, or supervises an oxygenate blending facility, or who owns or controls the blendstock or gasoline used or the gasoline produced at an oxygenate blending facility." Under § 80.76(a), registration is required for "any oxygenate blender that produces any reformulated gasoline." In addition, the refiner-oxygenate blender would be required to demonstrate compliance with the oxygen standard for the RFG produced in its oxygenate blending capacity separately from the RFG produced as a refiner. (10/31/94)

This question and answer was posted at Consolidated List of Reformulated Gasoline and Anti-Dumping Questions and Answers: July 1, 1994 through November 10, 1997 (pdf) (333 pp, 18.17 MB, EPA420-R-03-009, July 2003)