Black cod is a pacific fish, and not like other cod particularly, It's also known as sable.
1. Scale (they need to be scaled, whoever told you otherwise was wrong). Rinse
2. Slit the belly, rinse
3. Cut behind the gills, parallel, lean into the bone
4. Turn the fish, cut behind the gills, parallel, cut through the bone.
5. Remove the head, fins, internal gill structure, lungs, guts. It should all come as one piece.
6. Discard the soft parts, save the head and fin for stock.
7. Rinse the cavity, throurghly; use the point of your knife to make sure it's clean.
8. Trim the dorsal fins and ventral fins. Reserve for stock.
9. Cut all the way down the back, from head to tail, using the backbone as a guide. Remove the fillet.
10. Turn the fish, repeat. Rerserve the backbone for stock.
11. The rib cages will have been cut through at the backbone and stayed with the fillets. Remove them intact from fillet.
12. Pick the pin bones using tweezers or pliers. Black cod pin bones are large and determined.
13. Skin if desired, by pulling the fillet, head to tail, against a "wiggling" knife.
14. Portion the fillets as desired.
I use a 7" chef's knife for the first part, and a suji for steps 13 and 14. It's my take on the Japanese deba/yanagiba combo. You can use whatever you want, as long as they're sharp.
Don't hesitate or make tiny or indefinite strokes when fabricating fish. Even if it's not something you do often, do it assertively. Otherwise your cuts will be ragged.
Black cod (aka sable) falls apart very easily when grilled. Oil the heck out of the grill and the fish. If you're using a basket, oil the heck out of that too. Grilled, pan-roasted, or whatever, sable can overcook very easily if you don't pay attention. So, pay attention. Yes, you.
Sable might well be best known as a smoked fish, and is certainly good smoked, whole.