Conserning your question, it depends per player. There is not just a one-fix for getting up a level. Mostly, just check out your own (standard) games and analyse where the mistakes are. From that point forward, you can focus on where you need to improve.
People saying blitz is OK, or it isn't, or tell you to focus on your endgames... It's just so subjective. Playing blitz (seriously) has some merits to improving your tactics, or opening knowledge. Focussing on endgames also has merits, if your weakness is there.
Like I said, just analyse your own normal games thoroughly and see where your mistakes lie. F.i, you have a hard time to make a middle-game plan. Then focus on that by using relevant books (i.e. Grooten's Chess Strategy for Club Players, or whatever).
People saying blitz is OK, or it isn't, or tell you to focus on your endgames... It's just so subjective. Playing blitz (seriously) has some merits to improving your tactics, or opening knowledge. Focussing on endgames also has merits, if your weakness is there.
Like I said, just analyse your own normal games thoroughly and see where your mistakes lie. F.i, you have a hard time to make a middle-game plan. Then focus on that by using relevant books (i.e. Grooten's Chess Strategy for Club Players, or whatever).