Honestly, not dying from Barbarian Rage wearing off seems a little cheesy.
In what I understand about Barbarians, and the sort of primal rage they tap in to within their own willpower -- it can and should kill you. If you are barely hanging on by a few threads in the first place (unconsciousness) and you were forced into bleed out, your rage is still coursing through your veins. Once that burst of energy goes away, your body is no longer able to sustain itself beyond the limits it was just pushed (CON alteration), so you die.
If you are unsure you can take your current opponent at the start of combat with rage on, then don't rage. If you think you can beat someone with that extra edge, rage. Using rage at the start of combat in a sticky situation is all but bound to result in death. Forcing your body to go beyond its limits, in the sense of fantasy and even in real life, is always a risk. I don't see why this should be changed.
Blood frenzy sounds neat, as an aside.