Is Blue Cross your insurance carrier? Is it a covered procedure? Do you have deductible to meet? Do you have a co-pay?
If you're a Blue Cross member, and he's a Blue Cross provider, it's my understanding that they have to bill it. If for no other reason than they might not know what the contracted amount is for the procedure you had done, so they wouldn't know what to charge you. (Even with a high deductible, where you KNOW you will have to pay, they may not know what to charge you.)
Once it goes to Blue Cross, you can always come back and pay cash for the portion you owe. If they can even accept cash.
Not all offices have the ability to process cash payments. I'm not pointing fingers or accusing anyone but there have been some offices where the payment gets posted but the cash disappears.
There are many different scenarios that it COULD be. But I really appreciate your willingness to pay for services at the time they were rendered! You're right, healthcare might be in an entirely different place if everyone were so conscientious!