To what extent can Blanches flawed character be blamed for her downfall in a street car named desire?

Quite a lot. It was her actions that lead to her own downfall. After her husbands death (Allan Grey), any happiness that Blanche had once contained was lost, and she began to spiral downwards. (She blamed herself for his death because she told him that she saw his affair and that he disgusted her). In order to feel happy and desired again, she turned into a flirtatious maiden. She had an affair with one of her students at school, (she of course was fired when the school board found out), spent many nights with the soldiers that lived down the road from her home, and also spent nights at the Flamingo Hotel, having affairs with any man she cast under her spell. After losing her home and continuing with her many affairs, she was kicked out of towns people couldn't handle her anymore.

The remainder of her downfall was when she was at her sister's (Stella) home. Because she didn't want people to find out about what she had done in Laurel (hometown), Blanche told many lies to make to cover the truth. She continued to flirt with many men, including Stanley (Stella's husband), who wasn't fooled by her lies. When he found the truth about what she had done, he immediately told Mitch, his best friend who, at the time of the discovery, had been seeing Blanche. When Mitch confronted Blanche about what he heard, whatever was left of Blanche's sanity vanished. She couldn't tell what was real anymore and came up with some fantasy that some old lover was coming to pick her up and take her on a cruise.

Since she had told so many lies before, no one could tell if anything she was saying was the truth anymore. Because of this, they sent her to an asylum.

While Blanche was the main cause of her downfall, Stanley also played a slight role in it. He was the one who dug around for the truth behind her lies, he was the one who called her out on it. However, had Blanche treated him differently, he might not have done what he did.