I've had a blistering itchy rash behind my knees for 4 days now. the blisters have started to weep with a blood-like crust forming. is it healing?

Hard to say: As without have a better understanding of what may have caused this, in terms of possible contact irritants or allergens, versus an infectious cause(like bacteria or a viral issues) and even just the appearance, it's difficult to guide you. You may want to consider a formal consultation(a HealthTap Prime) consultation where you can upload an image or better detail the history around this rash.