Wait, so was Stanley attracted to Blanche?

I think he was. Blanchet also mentions that possibility, but it was probably another one of her fantasies. I think Stanley sometimes tried to impress her. And he was in an awfully good mood the beginning of the night his wife went into labor and he was alone with Blanchet in the apartment. But then he notices how delusional she is, and rapes her knowing it won't have too much consequences.

I mean...maybe you could argue that on a deeply deeply subconscious level he was, but I highly doubt it factored into the rape or his behaviour towards her at all. That, to Stanley, was all about putting Blanche in her place and reasserting himself as "top dog" and the one in charge of what he considered to be his household and quite literally stripping away the last remaining shreds of her dignity. The thing is that both Stanley and Blanche are completely accurate in their assessments of the worst aspects of each other: Stanley is proved to be right about Blanche is deluding herself and engaging in manipulative behaviours, while Blanche is proven to be right about Stanley constantly behaving little better than an animal and instinctively reacting with violence when things don't go his way. The rape was simply the logical culmination of that. We're talking about a man who beats his heavily pregnant wife on what's implied to be a fairly regular basis; that kind of man doesn't need any amount of justification of "I'm genuinely sexually attracted to this woman" for raping his mentally and emotionally unstable sister-in-law while his wife is giving birth to their first child. He saw an opportunity to put Blanche down in the most definitive manner he could and he took it; it's as simple as that. Rape being about power means exactly that: the excuse of genuine attraction rarely factors into it; it's an act that's all about the rush of exerting total control over another person.