Anytime when you are overwhelmed with your emotions that you can't see reality for what it is, that is basically the meaning of the term "blinded by your defilements". When you get caught up in doing something that you know inside is unbeneficial but you can't resist because your desire is so strong or your aversion is so strong, these are times when you are blinded by your defilements. Defilement basically means an unbeneficial quality -- unbeneficial actions, speech and thoughts, all of these are defilements. To defile means to make something dirty or stained and so on, so it has a negative implication. So when any of your defilements, your unbeneficial qualities arise and overpower you, we can say you are blinded, because you can't really see things clearly and you get swept away and do something unbeneficial.