Blue Cross Blue Shield insurance covers ER visits usually as long as the ER visit is a medical emergency and you really need to visit the ER.
If you can prove to Blue Cross and Blue Shield that the ER visit was medically necessary as in being an emergency then the Blue Cross Blue Shield will usually cover the cost of the ER visit.
Such as if you had a heart attack, were deathly ill, got into a car wreck, other accident etc and it was considered a life or death situation then Blue Cross Blue Shield will usually cover the cost of the ER visit.
Even if Blue Cross Blue Shield insurance won't cover the ER visit the emergency room will still see you and treat you and then work out the payment details later on.
But it's a good idea to get private health insurance in the event you need to go to the hospital as going to the ER or the hospital in general can be very expensive.