nomodeset doesn't fix black screen on startup

I'm trying to install Linux Mint on my 2010 21.5" iMac. I managed to install it after booting from the usb drive with the nomodeset option, but I haven't been able to boot from the actual install so far. The typical recommendation I found from googling is: on the boot selector screen (I'm using rEFInd) hold Shift and select grub options, then press e to edit the boot options and add nomodeset after where it says quiet splash. However, this does not work for me - I still get a black screen when it tries to boot up.