I've had 20 years martial arts experience and for a blood choke to be effective, both carotid arteries have to be constricted. If it's done well, the person can be unconscious in 10 secs, or less. If the choke is maintained, after 40 secs, there is a risk of permanent brain damage. Having said that, any trained person should know how dangerous any pressure applied to the neck area can be and should be aware of the amount of time that they are keeping someone in that position.
I don't see a blood choke here. I see a guy full of drugs whose heart gave out under stress.
I feel bad for saying this, but if it was a white guy, I would have almost zero sympathy, and sadly, for George Floyd, his lifestyle seemed to be on a collision course where it was most likely going to end badly.
What makes the cop look bad here, was a lack of duty of care. George Floyd was not moving, or resisting, and the cop should have checked and rendered first aid.