Princess+

Sirius didn’t know that the Dursleys were abusive.

At the end of Prisoner of Azkaban, he automatically assumes that Harry would rather live with them than with him. In the Goblet of Fire, Harry writes to him about Dudley’s video games and diet habits but never explicitly mentions anything about being abused. Even when he gets to Grimmauld Place in the Order of the Phoenix, he expresses his anger at not receiving information and though we know that he was also emotionally exhausted from spending another summer at an abusive household, he never discloses it. In fact, seeing as how he protected Dudley, Sirius could just as well be under the impression that the two were very close.

And I hate this so much because Sirius Orion Black grew up in an abusive household and if he had known for a single second that his godson was facing the same thing that he had faced, he would have really blown up a street in an attempt to get to Privet Drive and rescue Harry.