I was going through apocrypha with my level 66 "dead is dead" character

for the Sallow Regent black book quest and fell off a poorly lit narrow bridge

into the black gunk and was transported to the black book location instead

of having my latest save reload as has happened when my one of my characters

dies in game.

I wanted to get some opinions on if what I suffered in apocypha was an actual

death like in the real game world, or not.

It doesn't matter that much, as I was going to retire this character anyway since I

got through the main quests for the main game and dawnguard and dragonborn.

I just wanted some opinions about falling into the sea of gunk in apocrypha and

if it is considered actually being killed or just sent back to the "real world".

Well, lore-wise, your character isn't dead. Black Books are just that: books. Your character was, quite literally, just caught up in Herma-Mora's story. Dying in Apocrypha is not dying in "real life" for your character, so I'd say you're good to go. It's not a real death.

Your character didn't die they simply were forced back into reality. Think of it this way, have you ever been really caught up in a book to the point where you have a sort of movie playing in our head? If you have then have you ever had someone do something to force your attention away suddenly bringing you back to the world around you?

Your character was simply caught up in the story so when you "died" your mind snapped back to the world around it (kind of like suddenly waking up before you die/fall in a dream).

NPCs will tell you that those who read the books are still standing there physically but the rest of them went to the book since you were literally caught up in the story world.

Yeah definitely no death there. I had the same thing happen to me, you're simply ejected from the book.