Okay, one more time since you didn't seem to get it the last 100 times. THE BLOOD DID NOT COME FROM THE NECK. Drops of blood from the vent will often get on various parts of the skin that has already shed.
This is a perfect explanation and makes perfect sense. The OP has inspected the snakes neck and saw absolutely nothing, yet you still keep insisting that it must be some kind of neck injury. If there was an injury, there would be evidence of it. Bleeding from the vent is extremely common during shed and explains the DROP (Emphasis on DROP) of blood on the shed skin.
Why is this so hard for you to understand?