Powerstones

[Langy]

If you are in a dungeon crawl and you might be ambushed at any time (enough to justify constant vigalance) than you ARE in danger.

Except you can't be ambushed if there are not, in fact, creatures ready to ambush you. Nothing indicates a Bless spell needs to know about a danger in order to respond to it, and nothing indicates it will respond to a danger you only think you are in. Therefore, it doesn't respond to percieved danger - which is what you are talking about, no matter how hard you deny it.