I've managed to brick two Pi5's trying to install the Bookworm 2.9.3 image.

I'm trying to build a CNC controller based on a PI5. I have a half dozen Pi's around the house as music servers, DNS agents, and process monitoring, so this shouldn't be this hard.

These are 4GB PI's with 32GB SD cards. They loaded base Bookworm, written from the Pi Imager, and I ran them that way for a bit, and then I attempted to install rpi-5-debian-bookworm-6.1.61-rt15-arm64-ext4-2023-11-17-1520 on them. When I attempt to run the os, the screen goes black and the LED flashes, then stays constant green. On the first one, I thought it was something I did, so I bought a second one and had the same issue occur. 

At this point they won't even run the standard boot loader. I expect the screen to go green on boot, but it goes black instead. Documentation says that the Pi's are fried, and I'd hate to see this happen to others.

Can anyone suggest what I can do to figure this out?
 

First of all this is the first time I can recall this issue emerging.
I & many others I have used the image mentioned by the OP with no issues.

The screen going black isn't an indication that the Pi is bricked, there is a setting config.txt that controls whether you get the "rainbow" screen or not.

Generally there are issues which using the Rpi imager to write the image. If using windows Balena etcher is recommended, on Linux balena etcher can be used, dd is another alternative or whatever image writer that comes with your system.

To be pendantic, the bootloader is contained on the RPi board. As you have not mentioned any messages about of an out of date bootloader I doubt this is an issue.

One thing you could try, as you say you just recently bought a RPi5, is insert a blank SD card, connect an ethernet cable and boot. If you have the most recent bootloader the RPi5 should attempt to load an OS to the SD card via the internet.

pimylifeup.com/raspberry-pi-network-install/

One point to note, you get far far better bang for buck using a second hand corporate PC that a RPi5.

Another option is to let the RPi5 go through the boot process, check your DHCP for any entries that may be the said Pi and try to ssh into it.