Saber Lash is affected by armor, and can be dodged blocked and parried.
From the Blood Queen raid guide:
Stack stam.There's also her Blood Mirror ability. She casts this on the tank, who is then linked to the person standing closest to them. The linked player then takes 100% of the damage the tank does, as unresistable Shadow damage.
As far as Frosts, it's difficult to tell you what to buy unless you link your armory so we can see what you're upgrading from. I will say that the Frost trink is a significant up from Black Heart.