Are you honestly saying that a defender should not be able to tackle someone he cant see him?
How was that a poor hit? It caused a fumble. Every tackle isnt one on one in the open field when you can wrap up and form tackle.
It sounds like you want him to tackle with his arms?
Tackles don't happen in slow motion. You cannot plan what part of your body contacts which part of his body. You are trying to get a hit on the guy the best you can.
Never said that a defender should not be able to tackle someone who can't see him. By all means he should, but it should be his responsibility to do it properly. The key is what constitutes a tackle and that is entirely determined by the rules of the game. I am not a rules expert and I don't know all the nuances but I am pretty certain that flagrant helmet to helmet hits are considered dangerous and not in the spirit of fair play.
If the definition of tackling is to run around willy-nilly at full speed and collide in whatever way it happens then sure, Harrison is not culpable; in fact it would be impossible to determine for sure if anybody truly were. The key is, as you said, "trying to get a hit on the guy the best you can"; I would just add a clause to that- "within the rules of the game" which should be implicit in "best you can". Illegal hits would fall in the can't category. It should be entirely the defenders responsibility to get himself into proper position to make a legit tackle and execute it right. If it were just simply about stopping the other guy then why are things like tripping, blocking below the waist, etc. illegal?