Did you watch the interview? Blake said it wasn't like she was instantly addicted, she took it over a period of time and gradually got addicted. It's not like she woke up next morning and regretted it, - she carried on using. Even if she was high or drunk at the time, Blake said himself that she took it over a longer period of time before becoming addicted. He can hardly take the blame for "her being a drug addict". Yes he was a bad influence but nobody forced her to take drugs all those times.
Heroin is usually something you get hooked on straight away, rather than during a long period of time. Of course only he and Amy know the truth and one of them is dead.