Can I apply a new card after I've rolled my dice? For example lets say I am two points off from getting a card after rolling the dice. Could I, or another player, play "Blessings of the Gods" to give me another roll? Or do all modifiers have to be applied before the roll?
Pretty much all modifiers must be applied before dice are rolled. There are things like a Luckstone that break that rule. But the card would need to implicitly state that
Backing up what Troymk1 said, there is a specified order to attempting check.
Attempting a Check
Determine which die you’re using.
Determine the difficulty.
Play cards and use powers that affect the check (optional).
Assemble your dice.
Attempt the roll.
Take damage if you fail a check to defeat a monster.
Blessing and other cards that don't define the check are played during "Play cards and use powers that affect the check" step. After that you assemble you dice and attempt the role. The details for the steps are on pages 10-12.
Like Troymk1 said there are a few cards that let you do something if you fail a roll (Luckstone, Greater Luckstone, Seoni's Celestial Role), but they are very specific about that situation. Some weapons let you re-roll if you fail. Other than that, you have to take what you get.
Choices have consequences... you make you choice of how much to roll, how much help you need, and then do it. You can't say, ouch that roll sucked, let me bless myself for one more die.
If you make a mistake... live with it and learn from it!
That's part of what makes the game great!
Motrax