My character with Lifesaving Block had 3 HP and was killed by a Zombie Mob card. I'm assuming it's because Lifesaving Block only looked at the base damage of the card being used and didn't realize my character would die to the boost from a mob being nearby. I'm certain that is unintended however.
Yes, it only looks at base damage cards. I believe the devs are intending to look into that in the future however, believe Jon's replied that in the recent past at least.
In this similar thread: http://forums.cardhunter.com/threads/lifesave-block-did-not-trigger.3293 You get Jon's most recent response: This has been discussed for some time, and we'll have to see how things develop.