Welcome
I don't know much about macbooks but I'd look for gfx-firmware for the solution.
Chrooting: You mean the arch iso gives you a screen and Obarun doesn't? That's weird because there shouldn't be a difference. Did you try the base image, it could be that the problem appears through the X. When you boot do you get a bootloader screen at all? After this do you get text and then it turns black? If so, I suspect you are using the JWM image.
In any case, if you add the obarun core repository to /etc/pacman.conf
[obcore]
SigLevel = Never
Server = https://repo.obarun.org/obcore
then install the keyring and obarun's modification of pacman
# pacman -Syy obarun-keyring pacman
replace pacman.conf with pa....pacnew and run
# pacman -Syy
# pacman -S obarun-install
then as user run % sudo obarun-install
default mounted partition is /mnt but you can edit this to whatever you like from the installer