Annotating "brilliant moves" and "blunders" in chess games
No - there isn't a way to auto-annotate for all moves. Strong players often disagree on how to annotate moves using these symbols, particularly when the move is a non-forcing, non-tactical move.

I'm not a programmer by any means, but your approach of looking at changes in the effective score is how engines point out blunders, e.g. a significant drop is caused by a loss of time, material or the failure to parry a threat, e.g. mate. Additionally, you might want to look at the evaluation of the position in a few moves time, assuming the best moves are played.

For example, in Ubisoft's Chessmaster, the blunder alert function pops up and says that you've made a blunder because there is a 3 move checkmate coming. Maybe you could contact a chess software developer for ideas.