The blinding sunlight defense gets brought up quite a bit on these forums. If we're being honest, it's a bit of a crutch for user error. If your vision is so impaired that you're turning into oncoming vehicles and creating accidents, then perhaps pull over somewhere and take a break. It's not a defense to knowingly drive blind. You are not obligated to continue driving.
When discussing "at fault", are we talking about a ticket or do we mean insurance?
Insurance providers use the fault determination rules to determine a percentage of fault. You'd be considered 100% at fault in this scenario. If we're talking about tickets, they aren't interested in assigning fault percentages. You either can be charged or not. Whether or not someone is charged or convicted wont necessarily change what happens on the insurance side of things. They are two separate processes.