OSX bless works for me with grub.efi in any hfsplus partition using the form bless --folder /xyz --file /xyz/grub/grub.efi (if the file paths are correct - I use GUI and drag the file icon onto a terminal commandline)
However it is not always necessary to bless. On Apple Mac, the UEFI spec will boot *.efi without needing to be blessed, and without refit, if it fits the naming convention in the UEFI spec. So a simple bootable package can be built.
http://www.uefi.org/specs/ - EFI System Partition Subdirectory Registry UEFI Specification Version 2.3 3.4.1.1 Removable Media Boot Behavior - Table 9. UEFI Image Types
If grub.efi is renamed to bootx64.efi or bootia32.efi and placed in fat32 or hfsplus tree /efi/boot, it will then be shown on the restart with Optiion key.
However to get an automatic default boot, probably requires the bless form with the --setBoot option.
For Apple Mac with 64 bit EFI - address@hidden:~/test$ tree efi efi `-- boot |-- bootx64.efi |-- bootx64.icns `-- grub.cfg