Maybe you misunderstood. Your idea that only half of the blade entered victim's neck doesn't explain why there is no blood on the other half or anywhere else on the kitchen knife.
It's reasonable and factual that there was a spray of blood while Guede stabbed and sawed victim's throat.
Can you explain what prevented the blood from getting onto the blade?
But if we apply that same logic, then why isn't the entire imprint bloody from the blood on the blade? I don't get it.