MacBook Pro start up fails: blue-grey screen

Specs: MacBook Pro 15", Late 2011, Intel i7, 500Gb SSD HD. Problem occurred while running iOS X 10.12 (Sierra) and iOS X 10.13.2 (High Sierra). 10.13 updated yesterday morning and the computer ran fine for the first 24 hours. Fortunately, my data is backed up and not a concern.

This issue: MacBook loads to show the Apple logo and progress bar, the progress bar loads half-way then the screen blinks to blue-grey. Fans are running quietly. Despite many attempts to circumnavigate this problem, the computer always hangs on this screen. When left on the blue-grey screen, sometimes the computer appears unresponsive (there is no cursor) for up to 30 minutes, and sometimes the computer restarts automatically after approximately 2 minutes only to restart again in a continuous loop. Either way, it does not progress to the log in screen.

The first time this happened, I took my computer to a local Apple Repair Authorized company. In their words, "they reinstalled the OS and ran all the updates and then it started fine". And they charged me $100. Ten days later, I have the same problem again. I don't want to spend another $100 to have them run software updates, so I'm posting here first. Help!

Troubleshooting steps I have taken:

1. Cleared PRAM and restart: No change.

2. Reset SMC and restart: No change.

3. Attempted to boot in Safe Mode: Progress bar now loads completely and sits at "fully loaded" for about 2 minutes before blinking to same light blue screen.

4. Booted in verbose mode: rapidly scrolling text for about 10 seconds, then the same light-blue screen. Unable to read anything (...whats the point of verbose mode again?).

5. Booted in Single User Mode: While most of this is above my head, a few things seem odd to me in the log. These include:

nx_kernel_mount:1371: reloading after clean unmount, checkpoint xid 4754, superblock xid 4748

...

disk1s1: device is not readable.

disk1s1: IO failed, error = 13.

hfs_mountroot failed: 13

In Single User Mode the computer loads until :/ root#. both the commands "exit" and "reboot" result in the blue screen.

6. Ran Apple Diagnostics (online): Returned no hardware issues on either the quick or deep scans.

7. Local Apple Authorized Repair company ran more sophisticated Apple Hardware scan: This flagged the video card for a possible problem, but a specific video card test returned no errors.

8. Ran fsck in single user mode: Returned no issues, all checks okay.

9. Booted from USB copy of High Sierra (from which I can successfully boot another MacBook Pro): I can select the drive from the startup manager. When progress bar finishes loading, it blinks to a blue-grey screen and then to a dark grey screen. This appears to match the background when booting from the installation disk/drive, but the window with "Install", "Disk Utility", etc. doesn't show. I'm totally confused.

Miracles!

The computer gods are fickle. I have spent about 10 hours troubleshooting today. While typing this post I left the computer running the "restart loop" and after about 5 restarts, it jumped to the log in screen. I have no idea what changed.

While I'm glad I'm no longer staring at the blue-grey screen, I suspect this problem will occur again and I'd appreciate any diagnostic advice. I think I will do a clean install of 10.13 and manually port over my documents/applications.