I usually give my cast iron a good coat of peanut oil before I blacken.
Coat the fillets with melted butter and then sprinkle on your seasoning. I keep my seasoning fairly light cause sometimes it's just too damn strong.
If you flip them too early they stick and/or fall apart. A well seasoned pan helps a lot but by the time you get done cooking all of the season will be burned off. You'll probably need to reseason it.