OK, so are you saying that you only grill on one side until it's medium rare, no blackening seasoning on the side closer to the fire, THEN add the blackening seasoning on (I guess) the top ungrilled side, then zap that side (and only that side?) in a skillet?
No blackening both sides? No grilling or skilleting both sides?
I often leave out details, sometimes purposely, because of possible lack of interest and/or tl;dr.... and more often than not, I just forget.
The most common cuts of salmon offered at the grocery store are fillets, supremes and darnes. The the top pic and the one you chose to quote is a supreme. The other is a fillet. Darnes are the steaks cut from the collar that usually contain bones.
The darnes are arguably the best flavor and best for plate presentation, but they're just more "work" to eat, and the others are just more "bang for your buck".
So, with that in mind. I use fillets and supremes the most. Both will either have the skin still attached or it's obvious which side the skin was removed from. Duh. Just trying not to leave anything out no matter how redundant.
On both cuts, I'll buy which ever looks the freshest and has the best color. I prefer skin on, but usually end up with skin off because of availability.
I put my black iron skillet in a "hot spot" on the grill.
Skin off - I drizzle the top with olive oil and put it on the grill (that side on the heat) on medium high. Depending on thickness, after 1-2 minutes I'll quarter turn it for grill marks, won't see them, but I know they're there, and for equal heat distribution.
After 1-2 minutes I drizzle the "skin side" with olive oil and flip it over. Drizzle a little bit more OO on the top side, to let it absorb and run off. I repeat the 1-2 minute quarter turn, then heavily coat the top with blackening season (Prudhomme's Blackened Redfish Magic). Lightly press down with my spatula and flip it in the skillet seasoned side down.
Constantly Move it around just a little with with your tongs or spatula, or shake the skillet a little back and forth to help prevent any sticking. After about 20-30 seconds, you should be able to tell by the change in smell, carefully slide the spatula under it and plate it. If it's not "crisp" or black, carefully put it back on for several more seconds. Too long and it's not ruined , but not what you want. Not long enough, the same.
I'm sure this is way more than you wanted to know, but I had time, wanted to give a you better understanding of my "why" not just how and I got started and couldn't stop.
Skin on - Same, except before I flip it over to the skin side, I drizzle it with OO and coat the skin with sea salt. It helps insulate heat and moisture, and just because it sounds more important than it is.
Imma gone shut up now. Hope that helps. I dint feel like proof reading for grammar, spelling or any other mistakes, it was too long.