I'm in the Clockwood Mines. The phony Black Talon are on guard. I set up Imoen for a back stab, and then, wham, the Guard hit a miner who exploded into chunks. I thought blue meant protected. Am I wrong?

The blue circle means that a character is neutral/innocent. Are you sure you haven't attacked the miner by an accident?

No. Imoen came across a guard in the mines. I pulled her back to wait on the rest of the team. (If back stab doesn't work, they are near by.) So Imoen was a good distance away. A miner came by and walked into the guard's covey hole and whap, dead miner. Human players have an override button to attack blue circle players. I didn't know characters in the game did.

A blue circle just means that the character is non-hostile to the player. It has nothing to do with innocence (that's a class, not an allegiance). If an npc is scripted to attack another npc, then you can only stop it by killing the npc with the script, the guard in this case. The loss of a miner is no big thing.

Innocent in the sense of not involved. i have seen no other guard kill any other miner so if it is scripted, it is a unique script for the guard. i assume the game has the guard strike if he can't move. A way to avoid miners acting as blockers even unintentionally.