Definition of "in the dark
What is meant by "in the dark" is "without information" or "blind". For example, you open a hand with 3 big blinds. A player raises you (3b) to 12 bb. You announce that you are calling the bet and tell him "I check the flop in the dark". That is, no matter what the flop is (which you haven't seen yet), you will check. You can also make plays in the dark, that is, without looking at your cards. For example, if you have a player on your right who opens too many hands and you want to punish him in position, you could 3bet him in the dark, i.e. without looking at your cards.
Are there any advantages to playing in the dark?
The advantages of playing in the dark are obviously very limited, if not almost non-existent. Some pros on TV have started this trend (especially the one of checking flop in the dark after a 3bet) and many have followed. But from a poker strategy point of view, it's not advisable. You are better off studying the situation carefully and making decisions after analysis, not playing blind.
