Ignorant mind and Buddha nature

I read in a Mahayana Buddhist book (insight into emptiness) 2 concepts that I find hard to reconcile: 1- Our minds used to be pure, only Buddha nature, and got contaminated with ignorance that led to attachment and it makes us cycle in this cyclic existence. 2- We all have and always had Buddha nature in our minds. My question is: how could a pure mind with Buddha nature get contaminated in the first place? I can't understand how "Buddha nature" could let itself be tricked by ignorance, sounds strange, sounds like a "Big Buddha failure"! Unless...Buddha nature means only a distant background on the mind, something weak, that has no power to act by itself, weaker than the aggregates, but it doesn't sound like that. (!?) Can anyone help?