Actually, it's "blessed be God," though it's used rather like "thank God."
"How are things?" "Baruch Hashem, everything's fine."
Actually, that's a special case right there. "Baruch Hashem" is often used as simply the answer to "How are you?" Usual superstitions (and yes, even in Judaism they're little more than that) make people uncomfortable about saying "My life is going great!" lest they curse themselves somehow. So they just say "Baruch Hashem," blessed be God (i.e. things are alright).
I've heard it said that a response to someone saying "Baruch Hashem" in answer to how he's doing is to say "I didn't ask if you were religious; I asked how you were." But that's a little tacky.