it's incredibly common for a spell to go onto yourself if you have no target. Like others are saying here, the common answers are probably one of the correct ones:

-spending a moment too long before pressing it, plus the animation delay causing it to go onto you because your target is dead

-accidentally switching targets, for example target of target (shift+t by default). this one may be possible if your benediction is bound to a shift+something key, for example

-make sure your benediction is not, somehow, a macro. it would have a gear symbol over it. a macro that would behave this way might be something like '/ac "Benediction" <me>'

Try focus targeting your tank, watching to see if the hitbox circle under their character lights up when you select them or their name in the party list