how tf is a <600 ELO player supposed to know what makes a move a blunder and what doesn't?
It depends on the definition of "blunder". Don't define it as a computer (every bad move). Define it like this: a blunder is a move that is obviously bad even for the player who plays it. For a player under 500, this is pretty much only when you just hang a piece. Spotting simple tactics comes later.
The problem is very weak players have a harder time knowing what they don't know. They could use some guidance on what they're doing incorrectly. In this forum there are many beginners who are very certain they aren't making horrible blunders but they lack the ability to judge that.
^ This
I always see people giving beginners the same useless advice like "Always check to make sure that your move isn't a blunder before you play it" and it's like, how tf is a <600 ELO player supposed to know what makes a move a blunder and what doesn't?