So I was trying to backup a Raspberry Pi image to a USB Attached Hard drive...

I was trying to type out dd bs=4M if=/dev/mmcblk0 | gzip > /srv/Volume_1/NAS/Backups/NAS_OS_Backup.img.gz

Instead, I entered by accident... "dd bs=4M if=dev/mmcblk0 #" and then hit enter by accident.

I was using as handheld blue tooth keyboard, and for whatever reason Raspbian was set to British keyboard layout (Limey bastards! ). Suddenly the screen began to get filled with random characters scrolling by.

I just want an assurance that I didn't corrupt my Raspberry pi OS installation by doing this? I have no idea what the "#" would signify to Linux. I checked the home folder, and don't see any files created with the name "#".

If I were to make a wild ass guess... this command would have simply displayed the image file data on screen via the terminal... as I think "#" references the terminal itself?

# is for comment, it's ignored along with any text following it

You are correct. It outputted the data it was reading to your terminal. Nothing else bad happened.

as long as you didn't spooge your of= parameter, then you just spammed your own display, and didn't corrupt or overwrite a device

This. OP may have b0rked his command but is not fuX0red.

Hit CTRL-C a few times. If your terminal is all corrupted with weird characters type in:
reset
and hit enter. The characters that appear on your screen may not look right as you type them