They thought it wasa hit to the head and he was given a 5, they reviewed it and deemed it a clean hit and he ended with no penalty at all.
Perfect example in support of video review for some penalties.
I can't fault a ref one bit for calling that in real time. It was a blindside/lateral hit and there was no way either ref could get an unobstructed view of the initial point of contact. You can see Blais extend his arm upward through contact and you can see Lucic's head violently snap backwards. The difference between 'no-doubt-penalty' and 'clean hit' is just a few inches on a play like that. It is unreasonable to expect any on-ice official to accurately see and process whether that is a clean or dirty hit.
I'm not a huge fan of video review in a lot of cases, but I think it is absolutely a good thing for plays like this. The league just can't functionally straddle the line between outlawing headshots and allowing open ice hits through the body without a review mechanism. The game is simply too fast and properly positioned refs are too screened to reliably make that call in real time.
Good on Blais for getting low enough to avoid the head, good on the Flames for responding, good on the league for giving refs the tools to get it right, and then good on the refs for getting it right.