Linux in the alpha currently relies on the boot-time framebuffer, meaning there is no support for switching resolutions, VSync, hot-plugging, or anything like that. We have a driver for that (DCP), and it will be merged and released soonish, but first it needs a bit of clean-up and making sure it works on all the laptops too. That will make monitors work as you'd expect, with resolution switching and scaling and VSync and standby mode and all that jazz (and brightness control on the laptops). And as soon as the physical bits to switch the Type C ports to DisplayPort mode are ready (which is also being worked on as we speak), that will enable multi-monitor too.
The black screen you saw is probably because our bootloader (m1n1) failed to initialize video on the first boot. This is something we've seen happen; some monitors are temperamental and will do a reconnect cycle, and it's not clear why this happens (the internal DisplayPort-to-HDMI converter chip is probably also partially to blame). But the bootloader needs to kick off the screen and it needs to stick unattended, since at that point there's no real driver to handle hotplugging and all that. This is something that also affected Apple's bootloader (iBoot); it relies on the same interface to initialize monitors and what Apple put together is... quite primitive. Then in 12.0 Apple disabled that feature on the Mac Mini and they provide *no* boot-time display so now we have to use the same interface and take a guess as to what the best approach is. m1n1 is better than iBoot was with some monitors, and worse with others. We could make it more reliable, forcing it to wait several seconds for the output to stabilize, at the expense of increasing boot times for everyone... The good news is none of that matters once the Linux DCP driver loads, so the worst that an unreliable bootloader display will do is that your screen will sometimes stay black during the boot process until the real driver loads. Not great, but not a showstopper. But until DCP is in, yeah, a broken boot display means no display until you reboot.
We'll still take a stab at poking around and seeing if we can make it more reliable. It might come down to giving users a "flaky boot display mode" option on install, something where they can add 5 seconds to the boot process in exchange for handling these weird situations better.
Fun fact: when a display is not connected or just by default since 12.0, iBoot initializes a fake display mode that has the screen resolution of the iPhone 5.