I have tried to boot both ubuntu and arch on my iMac (21.5-inch, Mid 2010).  Both seem to boot, for example ubuntu makes its little bongo noise, and I can type zsh commands when arch boots.  But the problem is that nothing is displayed on the screen.When I boot arch or ubuntu, apart from a bit of text at the beginning of the boot, right before the screen goes blank.  The welcome to arch linux with the green [ ok ]s doesn't show up.  However, if I wait a while, about the time it takes to boot into the zsh shell, and type reboot, or shutdown -h now, nothing shows up on the screen, but the commands work, and my mac reboots or shuts down.I have a pc laptop with arch installed (it doesn't have UEFI though), and it seems like my mac screen goes blank when my pc screen would go blank and then the text would reappear a little smaller and better fit to the screen.  My mac is El Capitan (10.11) if that makes any difference.  I've tried booting with and without a partition ready for linux.  Is there any way to fix this, or is a mac from 2010 too old or something like that?  Any suggestions are welcome.
When arch linux boots on my mac, I can execute commands, but the screen is completely black
linux;boot;macintosh