does existentialism victim blame?

No, existentialism doesn't victim blame.

Victim blaming entails holding a person responsible for what befalls them. Blaming a woman for getting raped because she wore a short skirt is a good example of victim blaming.
But existentialism doesn't do that. It empowers the individual with freedom of what to do in a given situation.
Sometimes we may find ourselves in situations where there is no good option. Where we would have to choose the lesser of two evils and the best choice is still bad.
For example, imagine if you were trapped inside a cave and could not get out, but had access to a gun and preferred a bullet to the brain rather than starvation. Existentialism is not blaming the person for being in a cave - it's empowering the person to choose between a slow death or a fast one.

Or a more light-hearted example...imagine if you were holding a surprise-party for a friend and they noticed unusual activity and asked you about it. You may choose to tell them a 'white lie' just so you can do a greater good. Existentialism does not condemn you to hell for this.

Existentialism is what it is - truth. And empowerment. Make your choices to the best of your ability because these will define your life.