Yesterday I received the newest round of Manjaro branded Pinebook Pros shipped straight from Hong Kong - awesome!
Except it wouldn't boot out of the box, just a black screen Some digging around online led me to discover that some Pinebooks have their eMMC module disabled by a hardware switch from the factory, which prevents the device from booting. Changing the switch back to the on position correctly re-enables the eMMC drive which allows the OS to boot properly.
Fixing this requires taking off the back of the laptop to access the eMMC switch (just requires a mini philips screwdriver, and be careful of the attached speakers). Here is the pinebook dissassembly guide, note #24 for the location of the eMMC switch. Simply toggling it on (towards the back / hinge) fixes this and allows the laptop to boot normally.
I also created a short YouTube clip showing how to fix this issue: https://youtu.be/LWbncyt6xfg. Hopefully it helps others, since I had a little bit of trouble finding the eMMC switch at first!