A dark tunnel bluff is a repeated bet that is made as a response to the feeling that you must bet, without thinking about what the opposition are doing or why they have stayed in the hand.
It doesn't have to be a pure bluff, rather it refers to unneccessarily adding to the pot, often with a marginal hand with which a player doesn't know where he stands. Such bets do not have the desired effect of the opposition laying down their hands.
Typical dark tunnel bluffs are hands in which beginners raise with a good starting hand that doesn't improve, but they cannot bring themselves to stop betting even though the opposition makes no move to fold. They tend to happen when a player is on tilt.